Kate was on elegant form at the bash in a dress by one of her favourite designers Alice Temperley. The pretty outfit had a sheer black neckline and hem and was embroidered with white flowers.
She attended the event without her husband Prince William, who was on duty with the RAF.
Sports fan Kate looked thrilled to be welcoming the athletes, and was spotted chatting to her friends on the hockey team, who she visited in the run up to the Games.
Before she joined the sports stars, the Queen and Prince Philip met some of the senior figures who made the Olympics possible in the Palace's 18th Century Room. Among them were Prime Minister David Cameron, Health Secretary Jeremy Hunt - formerly Culture Secretary – and Lord Coe, chair of London 2012.
Mr Cameron said before joining the reception: "It was an amazing summer and I think people will cherish those memories, you know, almost more than the 1966 World Cup. I think people will look back and just think, ‘what an incredible summer’ - success after success, medal after medal and the whole country putting on its very best face for the world."
"What matters now is that we maximise the momentum and the legacy following the Olympics and Paralympics.
"We had a cabinet meeting about that today and Seb Coe was there, and we were talking about the physical legacy of East London, the important economic benefits we want, there’s the sporting legacy carried through to Rio and school sport, there’s also the legacy in terms of attitude to disabilities, which I think is a real chance of a step change in our country."
"But tonight it's very, very generous of Her Majesty to open up Buckingham Palace and have everybody in. I think it's a lovely finish to an amazing year for British Olympians and Paralympians."
Among the sports stars present were cyclist Sir Chris Hoy and Paralympian Eleanor Simmons, who took double gold in London to equal her previous success in Beijing. She tweeted: "1st Stop – Buckingham Palace! #superexcited"
One of the first to arrive was Para-equestrian rider Lee Pearson, who said: "I was driving down here and reminding myself that it had been a home Games and that made it a bit more special. Coming here to Buckingham Palace to celebrate a home Games is also special because so many of the Royal Family came out to watch us."
Bronze medalist swimmer Rebecca Adlington clearly enjoyed herself at the royal bash. She tweeted afterwards: "Had an amazing time tonight at Buckingham Palace with all the medallists from the Olympics + Paralympics! So nice to see everyone again!"