In the grounds of Windsor Castle, where the Queen was in residence on her 87th birthday, the pregnant Duchess spent time talking to the excited visitors from both the UK and the Commonwealth.
Kate, who is six months pregnant and fast approaching her July due date, also presented around 400 of the Queen's Scout awards.
The scouts and their guests gathered in the castle's sunny Quadrangle, eagerly waiting to catch a glimpse of Kate, who was Brownie in her youth, and start the parade.
In honour of Her Majesty the Queen's birthday the band of Irish Guards included a rendition of Happy Birthday in their musical programme.
From the Quadrangle the crowds headed to St George's Chapel for the National Scout Service and then moved on to the Royal Mews.
The day celebrates those who have given an outstanding contribution to Scouting as well as those who have achieved the Queen's Scout Award. The award is gained through a variety of tasks including a year's community service, a challenging four day expedition and a residential project.
The National Review of the Queen's Scouts, an event that dates back just less than 80 years, is annually held at Windsor Castle and as a Scout Association volunteer Kate was the event's royal guest of honour.
Elsewhere, Kate's brother-in-law, Prince Harry, was busy at the London Marathon where he presented the winners with their medals.