Britain's longest-reigning monarch and her life-long partner walked for part of the journey, before being driven past the crowds who came out en force to mark the day.
Well-wishers reportedly sang Happy Birthday as the queen passed them, with some offering cards, flowers and a slice of cake.
But it was left to Great British Bake Off winner Nadiya Hussain to present the Queen with her official birthday cake - an orange drizzle sponge.
At Tower Bridge, the day was marked with a 62-gun salute - one of many to mark the occasion.
The walkabout comes ahead of this evening's festivities, which will see the Queen light the first of more than 1,000 beacons across the country.