The trainee RAF helicopter pilot is more used to planning sorties and checking weather forecasts than legal argument.
But in the ceremony on Monday at London's Middle Temple the Prince was not only called to the Bar - to be made a honorary barrister - but called to the Bench, the Inn's governing body.
William received good wishes from his "proud grandmother", the Queen, who could not make the event as it clashed with another engagement.
The Queen regretted not being able to attend the ceremony but her words of support were read by Sir George Newman, the Middle Temple treasurer.
The Prince made his acceptance speech in the Inn's Elizabethan hall before almost 300 guests sat down to dinner and, in accordance with tradition, described his interest, hobbies and passions.
William told the law lords, senior barristers and law students associated with Middle Temple: "I play football, rugby and water polo, not very well, but particularly pride myself in being a quite magnificent armchair centre-back at football and flanker at rugby... and now you have made me a barrister - I promise not to practise, except for the odd speeding ticket."
The Prince is the sixth member of the Royal Family to be called to the Bench as a Royal Bencher and is following in the footsteps of the Queen Mother, called in 1944, and his mother Diana, Princess of Wales, called in 1988.
William, who wore a gown for the ceremony, told the guests about his activities during the past decade, describing how he spent a gap year in Chile and Africa, had the privilege of serving in the Household Cavalry and was now in the RAF training to be an operational search and rescue pilot.
Talking about his charitable interests, the 27-year-old royal said he wanted to do what he could to help society's "disadvantaged", adding: "I'm also very interested in initiatives in creating sustainable development in the light of climate change."
The Queen said in her message to her grandson: "I regret that I am unable to be with you this evening for your call to the Bench which follows in the tradition of Royal Benchers established at Middle Temple for my great-grandfather, the Prince of Wales, in 1861."
In a reference to the Queen Mother, who referred to herself as the "woman of the domus" - or house - after become a Bencher, the Queen added: "I should not wish this message to be an unwelcome intrusion into this evening's proceedings.
"However, as the daughter of the 'first daughter of the domus' and the proud grandmother of your latest Royal Bencher, I would simply ask that my warmest of wishes be extended to all present for a most successful and memorable occasion."
