The Duke of Cambridge kicked off the first football match to be played at Buckingham Palace - after warning that any players breaking windows would answer to the Queen.
Before kick-off, William, who helped to organise the match, shook hands with the players and met referee Howard Webb, one of the country's best-known football officials.
He has performed at the highest level, having taken charge of World Cup and Champions League finals and the recent Premier League Manchester derby match.
The two teams, who play in the senior division one of the Southern Amateur League, warmed up in the autumn sunshine before the match began.
Civil Service FC is the sole surviving club out of the 11 which founded the FA in the Freemasons' Tavern in Great Queen Street, London, in 1863 and later drafted the 13 original laws of association football. Polytechnic FC was formed in 1875.
The Civil Service team presented William with two tiny tops for his son, Prince George - one red and the other white, both with "HRH 1" on the back.
Earlier, the Duke paid tribute to football's unsung heroes by honouring 150 grassroots volunteers during a Palace reception.
William, president of the Football Association, presented the hard-working helpers with medals recognising their efforts, part of the FA's 150th anniversary celebrations.
He told the guests: "At its best, football is a powerful force for good in society. It binds people from different backgrounds, communities, faiths and abilities - and gives them a common interest, a unifying identity.
"I believe, over its 150 years, football has remained a wonderful example of the power of community and of our ability to come together to organise and to enjoy a simple pastime."
William joked that if a window was smashed, the footballer responsible would have to face his grandmother.
He said: "This magnificent home, Buckingham Palace, is at the heart of the nation, and so there cannot be a more fitting setting to celebrate our national game, and to celebrate all of you."
Speaking about the Queen, he added: "One warning, though: if anyone breaks a window, you can answer to her."
William watched the first 20 minutes of the hard-fought match, with both teams giving their all.
The Queen's lawn is more used to being trodden by garden party guests in their smart shoes and stiletto heels than footballers with their boots.
But the grass, which had been re-seeded in parts after a busy summer of events, held up well to the sliding tackles.
There was friendly banter between some of the supporters and the referee Mr Webb, with one spectator shouting at the officia,l who has a shaved head: "Get the hair out of your eyes, ref."
At half-time, Polytechnic were leading 1-0 but their half-time refreshments will probably never be better served.
Palace footmen and women wearing their uniforms of tail coats and scarlet waistcoats, carried paper cups of water, orange segments and Mars bars on silver-plated platters for the players.