It will be the most important day of their shared lives to date, imbued with centuries-old tradition as they take vows before God, their families and the Queen.
The couple, who announced their engagement in November, are planning their own wedding celebration that “reflects the characters of the bride and groom”.
Here's all the latest news.
When is the big day and who is planning it?
The wedding is on Saturday, May 19 at St George’s Chapel in the grounds of Windsor Castle, where Prince Harry was christened. It is said to have become a "very special place" for the couple in their relationship so far.
The Lord Chamberlain's Office at Buckingham Palace is responsible for organising royal weddings, but Harry and Ms Markle are leading the show. However, under the guidance of Harry's private secretary, Edward Lane Fox, the Lord Chamberlain's Office is dealing with the ceremonial aspects of the day.
The service will begin at 12pm.
The "core aspects" of the wedding, including the church service, associated music, flowers, decorations and reception, will be paid for by the Royal Family, with Ms Markle’s parents both set to attend.
How did Prince Harry propose?
Former Suits star Ms Markle told last year how she could not wait to say "yes" to the prince when Harry got down on one knee as they cooked a roast chicken dinner at his home, Nottingham Cottage in the grounds of Kensington Palace.
Indicating the depths of his feeling for his bride-to-be, the prince revealed in a television interview: "It was this beautiful woman just sort of literally tripped and fell into my life, I fell into her life."
Invitations - who is on the guest list?
Invitations have been posted to 600 lucky guests, with a select 200 close friends invited to an after party at Frogmore House, Windsor.
The couple do not have an official list of political leaders on the guest list which means neither Prime Minister Theresa May or US President Donald Trump will be going, and former US President Barack Obama has also missed out.
Royal sources said the guest list will be restricted to those who have a direct relationship with the couple, and was taken both because St George’s Chapel is relatively small, and because the prince is not in the direct line of succession.
Some members of the public have received an invitation to wait in the grounds outside the chapel to watch the bride and groom and their wedding guests arrive and leave.
Other guests will include members of Ms Markle's family - her mother Doria Ragland and father Thomas Markle Snr - and of course many members of the royal family. Tennis star Serena Williams and Ms Markle's former Suits co-stars Sarah Rafferty, Patrick J Adams and Wendell Pierce could also be on the list.
The invitations were produced by Barnard & Westwood, which has held the "Royal Warrant for Printing & Bookbinding by Appointment to Her Majesty The Queen" since 1985.
Die-stamped in gold and then burnished, the invitation features The Three Feather Badge of The Prince of Wales. They also reveal that the dress code is Uniform, Morning Coat or Lounge Suit, or Day Dress with Hat.
What about the best man and bridesmaids?
The Duke of Cambridge is expected to be Harry's best man, or perhaps jointly take on the task with one of Harry's childhood friends.
Prince George and Princess Charlotte look almost certain to be page boy and flower girl. The siblings already know the score - they performed those roles at the wedding of Pippa Middleton, the Duchess of Cambridge's younger sister, to James Matthews in May last year.
Ms Markle may also choose some grown-up bridesmaids from her close set of friends - most likely Canadian stylist Jessica Mulroney and Indian actress and philanthropist Priyanka Chopra. She might even pick a best woman.
Ms Mulroney and Ms Markle have been firm friends since meeting in Canada where Markle filmed several series of her Netflix series, Suits, and bonding over a shared love of yoga (the duo both practice regularly).
The dress - all bets are off
Bookmakers suspended betting on the designer of Ms Markle's wedding dress in March when British couture company Ralph & Russo pulled ahead to become 1/3 favourites.
The label, helmed by Australians Tamara Ralph and Michael Russo (a couple in real life) have shot to prominence after a string of celebrity endorsements.
In 2014, Angelina Jolie wore the label to Buckingham Palace to receive an honorary damehood, in 2016 Gwyneth Paltrow to the Oscars. Add to that list of fans Beyonce, Rihanna and, finally, Meghan Markle herself, who chose a £56,000 embellished couture gown for the official photos taken to mark her engagement to Prince Harry.
Before betting was suspended, London-based designer Erdem was at 3/1.
Ms Markle's stylist and close friend Ms Mulroney runs a bridal store in Toronto was said to have helped the bride choose her gown.
A very traditional ceremony
The couple will be making their vows at St George's Chapel, which last hosted a royal wedding in May 2008 when Peter Phillips - son of The Princess Royal - married Autumn Kelly. Prince Edward also wed Sophie, the Countess of Wessex, there in June 1999.
The traditional ceremony will be conducted by The Dean of Windsor, The Rt Revd. David Conner while the Archbishop of Canterbury, Justin Welby, will officiate as the couple make their marriage vows.
Miss Markle, who was a UN women's advocate, is unlikely to opt to obey the prince. They will probably choose the Series One (1966) Book of Common Prayer ceremony, just as the Duke and Duchess of Cambridge did, which allows the bride to drop "obey him" and "serve him" from the religious proceedings.
Harry also has to decide whether to wear a wedding ring. William does not wear one.
The regiments Prince Harry served with in Afghanistan will also have a “special place” during the ceremony. Household Cavalry troopers will line the staircase at St George's Chapel, while streets within the precincts of the castle will be lined by members of the Windsor Castle Guard from 1st Battalion Irish Guards, and by Armed Forces personnel from the Royal Navy Small Ships and Diving, which has the Prince as Commodore-in-Chief, and the Royal Marines, where he is Captain General.
The 3 Regiment Army Air Corps, where Prince Harry served as an Apache Pilot in Helmand Province, Afghanistan, will also be represented, as well as The Royal Gurkha Rifles, his comrades in Afghanistan in 2007, and RAF Honington, where he is Honorary Air Commandant.
Don't miss the procession
After the ceremony, among the first people to congratulate the couple will be some of the 2,640 charity workers, community champions and local school children who are being invited to watch the wedding from inside the walls of Windsor Castle.
Then at 1pm, the newlyweds will then undertake a two mile tour of Windsor greeting well-wishers. They will leave Windsor Castle by carriage via Castle Hill, travelling along the High Street and through Windsor Town before returning along the Long Walk for their reception.
Not one, but two receptions
Following the carriage ride, the congregation will await the happy couple in St George's Hall, for the first of two receptions.
The first, more formal afternoon reception for some 600 guests, will be hosted by the Queen. The 180-ft long room, traditionally used for state banquets, seats up to 162 people and was redesigned in a modern Gothic style – with walls lined with suits of armour – following the fire at the castle in 1992.
The second evening reception, for 200 close friends and family, will be thrown at Frogmore House by the Prince of Wales. Standing about half a mile south of Windsor Castle in Windsor Home Park, Frogmore House has been a Royal Residence since 1792. It is where Harry and Ms Markle posed for their official engagement photos in November.
The location of the venue - away from the Castle - will offer the pair privacy as they celebrate with friends and family.
Scant details have emerged about the receptions so far, but we do know that the couple have asked Claire Ptak, an east London pastry chef, to make an organic lemon and elderflower wedding cake.
Kensington Palace said Ms Ptak, owner of the Violet Bakery in Hackney, will create a cake for the Royal wedding incorporating "the bright flavours of spring". It will be covered with buttercream and decorated with fresh flowers.
It is believed to be the first time a Royal couple have eschewed tradition by not serving a fruit cake to their wedding guests.
The Duke and Duchess of Cambridge chose Fiona Cairns to make their 2011 wedding cake, a traditional multi-layered fruit cake with a floral design.
The couple have chosen high society florist Philippa Craddock to design their day, using seasonal plants and flowers from the Crown Estates and Royal Parks chosen particularly for their pollinator-friendly properties.
Aisles, pews, tables and halls will be lined with plants from wildflower meadows, picked to “provide a great habitat for bees and help to nurture and sustain entire ecosystems by promoting a healthy and biodiverse environment”, Kensington Palace said.
Ms Craddock has been named “Queen of London florists” by British Vogue and “a floral mastermind” by Tatler.
Peonies are one of Ms Markle's favourite flowers, so it is likely her floral bouquet will include these fragrant blooms, which are in season in May.
The prince will be putting pen to paper to write his speech for their reception. But Ms Markle, who is an accomplished public speaker, is expected to break with tradition and deliver her own as well, offering “affectionate” praise of her new husband, thanks to the Queen and a few suitable jokes.
The former actress, is “ready” to speak at the reception, should her father Thomas Markle choose to avoid the limelight, the Sunday Times reported earlier this year.
Mr Markle, an award-winning Hollywood writing director who now lives a quiet life in Mexico, is expected to walk Ms Markle down the aisle, but may not make a formal speech.
At the wedding of the Duke and Duchess of Cambridge, reception speeches included words from best man Prince Harry, the groom’s father the Prince of Wales and Michael Middleton, who affectionately described how he knew his daughter’s relationship was serious when her boyfriend landed a helicopter in his garden.
Will the Spice Girls play?
In February, one of the five original Spice Girls Mel B hinted that the Spice Girls were expected to reunite for the Royal wedding. The singer, known to fans as "Scary Spice", claimed she had received a "proper" invitation during an appearance on a television chat show.
Fellow Spice Girl Victoria Beckham also appeared to let slip she was attending the big day during an appearance on The Late, Late Show with James Corden earlier in April. Asked if she would be among the guests, she said: "I, uh, I, I don't know..."
Victoria and her husband David were invited to the 2011 royal wedding.
New royal titles
Ms Markle will become an HRH and a senior royal after marrying Harry. She is also expected to become a duchess, as Kate Middleton did when she became the Duchess of Cambridge after saying her vows in 2011.
By tradition, male members of the Royal family receive a title from the monarch on their wedding day, and the vacant title Duke of Sussex is regarded as the most likely choice for the Prince, meaning Ms Markle would become HRH The Duchess of Sussex.
Like Prince Harry, the previous Duke of Sussex lived at Kensington Palace, and he also married for love, rather than choosing brides who fitted the traditional royal mould.
Will there be a bank holiday?
The wedding is on a Saturday, so there will be no new bank holiday declared.
Although the public were given a day off to celebrate the weddings of Prince Charles to Diana Spencer in 1981, Prince William to Catherine Middleton in 2011, and Princess Anne to Mark Phillips in 1973, no such national holidays were awarded for Prince Andrew or Prince Edward.
However, it's not all bad news because pubs and bars will be allowed to do away with the usual curbs on opening hours on the wedding weekend.
Ministers plan to extend licensing hours across Britain to allow venues to remain open until 1am on the nights of Friday 18 and Saturday 19 May - which is also the day of the FA Cup final.
There will also be lots of street parties planned across the UK. For information about how to host your own, click here.
Souvenirs and wedding china
The royal bride and groom-to-be have approved a range of commemorative china to celebrate their nuptials.
The exclusive design in white and cornflower blue has a monogram of the couple's initials at its centre, tied together with white ribbons and surmounted by the coronet of Prince Harry. The decorative border of each piece is inspired by the ironwork of the 13th-century Gilebertus door of St George's Chapel and each is finished with 22-carat gold.
The Royal Collection Trust range, which is made in Stoke-on-Trent, includes a decorative miniature coffee mug, coffee mug, pillbox, tankard and plate.
The Royal wedding souvenir phenomenon dates back to Queen Victoria’s wedding to Prince Albert in 1840, linked to the rise of mass production in Victorian Britain.
It remains a thriving industry today, with an estimated £222 million spent on memorabilia in the run-up to the 2011 royal wedding, according to the Centre for Retail Research.
And finally... the honeymoon
Harry and Ms Markle will be hoping their chosen destination stays top secret as they enjoy a break after the busy run-up to the wedding.
Namibia has pulled ahead as the odds-on favourite. The southern African nation is best known among travellers for the enormous Sossusvlei sand dunes of the the Namib Desert, but reports suggest that the couple are looking instead at a safari trip to Hoanib Valley Camp in the country's north west.
Botswana, where they holidayed in the early days of their whirlwind relationship, is also a good bet, (Ms Markle's engagement ring features diamonds from Diana's personal collection, and a stone from Botswana), but wherever they go, the newlyweds will be looking for seclusion away from the limelight as they start their married life together.
Other destinations could include Tuscany, Hawaii, Turkey and the Philippines.