Meghan, Duchess of Sussex said she was “glad to be back in UK,” as she gave her first public speech in two years.
“It is very nice to be back in the UK and very nice to be here at One Young World,” she said at the event - which is a gathering of 2,000 young leaders from across the globe.
The launch party at the Bridgewater Hall will be followed by events and talks until Thursday, with Bob Geldof, Andy Burnham and Dame Sarah Storey also involved.
One Young World is open to youth leaders of climate-conscious organisations.
“The resounding spirit I believe you will hear is that you are the future,” the Duchess continued, “But I would like to add to that, [I would add that] you are also the present.
“You are the ones driving the positive and neccessary change needed across the globe now in this very moment.”
Ms Markle has been at One Young World several times and used her experience to try and inspire the next generation.
She added: “It was several years ago in 2014 that I was first invited... I was probably a lot like each of you, young and ambitious - advoacting for what I believed in. I was wondering, as well, ‘how did I get here?’
“I was thinking, ‘I’m the girl from [US television series] Suits and I was surrounded by world leaders, humanitarians and prime ministers....
“But One Young World saw in me what I wanted to see fully in myself. They saw in me, just as I see in you... The present and the future.
“You here in this present moment, this is where it is all beginning.”
She went onto pay tribute to the “courage” of a woman from Eritrea who fled her country and persevered “against all odds”.
Although Prince Harry also attended, he did not speak alongside her.
“Your time is now, and the important work can’t wait for tomorrow,” she finished.