Named after the John Lennon song Number 9 Dream, the company will be co-managed by Hannah Bentley, a former group account director at JWT and DDB, and the commercials director Thomas Krygier, who has produced work for the British Army, Cadbury's Flake and Citroen.
The director Iain Cadby is the first to sign up to Number 9 Dream's roster.
Cadby, a former designer from Why Not Associates, worked on The Economist, Virgin and Sony.
Fenton said: "We intend to build a diverse company, not so large that we cannot manage our talent and our agency relationships effectively."
Fenton was part of JWT's management group and ran three departments.
At Simons Palmer, he worked on Nike, Wrangler and BT. Before moving into advertising, he worked in feature films.
Since 2003, Krygier has been working in the US and his native Germany.
Originally signed in the UK by Tony Kaye, he then joined Arden Sutherland-Dodd before launching his own company, Krygier Hirshckorn, with the producer Nick Hirshckorn.
"I'm excited to build a company on the belief that creativity can be inspirational," Krygier said."There are too many people in the business whose motivation is not the quality of advertising."