Williams co-founded Fishburn Hedges and returned in 2011 as group director of strategic planning following a period spent working in various non-executive roles. He said he was planning to semi-retire, but would continue to take on some advisory work.
He will leave at the end of the week, and then take some "overdue" holiday.
"The job is never quite done, but after two years of providing a supportive role I have a hankering to get back to the diversity of portfolio life," he said.
"I’m leaving the teams in good shape. No one ever quite retires, and I plan to get a portfolio of some advisory work, but nothing formal, and this is my last big job."
Fishburn Hedges recently appointed former deputy director of think-tank Policy Exchange David Skelton as a senior adviser.