The photos show the prince wearing an England football shirt as he plays in the garden of his Kensington Palace home and a third snap of him looking more thoughtful on a family holiday.
Followers on social media were quick to comment how much George resembles his father at the same age.
Wow, he looks exactly like his dad! Happy 6th Birthday, Prince George! 🎂🎉🎈— Toni🐶🐱D's are in the House! (@mulatta10) July 21, 2019
Happy Birthday Prince George! You look so much like your Dad in the first picture. Have a great day!— Colleen Walsh 🇺🇸 (@cwalsh1215) July 21, 2019
Happy birthday Prince George 💕 you look more like your dad everyday!— MaraHime (@MaraofRoses) July 21, 2019
One Twitter user said ‘Wow, he looks exactly like his dad!’ while another commented ‘you look more like your dad everyday!’
Prince William attended the pre-prep Wetherby school in Notting Hill in 1988, when he was six.
Pictures from that year show his serious side as he attends Trooping the Colour with his parents Prince Charles and Princess Diana and his younger brother Harry.
But others show his more playful side, as he played a starring role as page boy, dressed in pink and white, at the society wedding of Camilla Dunne and the Hon Rupert Soames.
Prince George has often been photographed looking more pensive than his younger siblings Princess Charlotte, four, and one-year-old Prince Louis.
They said: "Yes, he has been protected from some of the realities about being third in line to the throne, but it is almost as if he is hard-wired to feel a sense of responsibility.”
They added that once George becomes comfortable in a situation he turns “playful” - a side royal fans have been delighted to see at public events.
Kate recently took her three children to watch William and Uncle Harry play in a polo match and George and Charlotte were spotted kicking a ball around.