The Royal pair made an appearance at the annual ICAP fundraiser on Wednesday, becoming city traders to raise money for various charities.
During the day the mother-of-two was speaking to a broker, Andrew, whilst an eavesdropping William leant across and told her; 'Catherine, I think you can stop flirting with them now.'
The Duke and Duchess manned the phones as part of ICAP's charity fundraiser
The Prince, 33, then turned to another trader, Brett Lincoln, and, raising an eyebrow said; 'Sorry my wife is just embarrassing me.'
It is not known whether Kate's persuasive techniques paid off by closing a deal, although her new shorter haircut was certainly a hit with the crowds.
The Duchess of Cambridge wore a recycled LK Bennett green dress, which she was last seen in at the Queen's Jubilee Tour in 2012.
The jokes continued throughout the day, leaving other employees on the floor in fits of laughter, with William saying of Kate; 'Hmmm, I think there is rather more haggling going on over that side of the desk.'
The Etonian also found himself on the phone to an old classmate, Vachan; 'Vash, you old dog! How are you? What are you doing? Clearly you're not progressing with your career... is this what you did with our maths at school?'
The Duchess of Cambridge spoted a brand new shorter hairstyle for the event
Later the parents played a game of table tennis and in scenes reminiscent of their abseiling trip in Wales last month, the pair's natural competitiveness was obvious.
William said; 'Come on, let's try and have more than a rally... you're not normally this polite when we play.'
With his wife responding; 'Or quiet... '
As well as inviting celebrities to join them for the day, the ICAP fundraiser sees employees wearing fancy dress, and one outfit certainly made an impression on Kate.
Nigel Halligan, wore stiletto heels and no trousers, told the Duchess; 'I really don't know how you women do it!'