David Enthoven, who lost his battle with liver cancer earlier this month, managed Williams since he quit Take That in 1996, and guided him through a series of highs and lows, including a run of UK chart-topping singles, hit albums and a battle with addiction.
Following his death, Williams tweeted: "My Friend, Mentor and Hero passed away today. David Enthoven, I love you. RIP."
The music mogul was remembered at a star-studded service in Chelsea, where more than 700 people showed up to pay their respects, including Williams, who reportedly paid a musical tribute to his late friend, according to Daily Mirror.
British singer Lemar also serenaded guests with a rendition of Jerusalem, while Lucy Pullin performed Williams' hit "Angels", accompanied by a full choir.