Now in its fourth year, the project supports influential people in championing a project of their choice.
Williams will be organising a series of music events in the run up to 2012 to showcase new talent and promote sustainability to Londoners.
Mayor of London Boris Johnson said: "I warmly welcome this latest batch of inspirational individuals, brimming with clever ideas and the leadership skills to work for the good of London over this coming year and especially in the lead up to 2012."
Williams added: "This is a huge opportunity to show Londoners that sustainability can be about fun and positive expression. I sometimes worry that we spend too much time on the doom and gloom when we should be concentrating on the benefits of sustainable living rather than seeing it as a chore.
"I can’t wait to start on my project and I look forward to collaborating with industry colleagues. If anyone has ideas they want to contribute then please get in touch with me through our website. It’s a huge honour for me and the rest of the team at Seventeen."
Past London Leaders have included designer Wayne Hemingway and chef Arthur Potts-Dawson.