The communications agency will take charge of the development and execution of a new brand strategy designed to increase the magazine's core audience.
The account, which was won without a competitive pitch, has yet to be allocated a definite budget.
Paul O'Neill, an M&B partner, said: "Time Out has a circulation of around 85,000 readers who are very loyal. But we'll be helping to develop a brand idea that will make the magazine more appealing to more people. Although Time Out is renowned for listings, there's a lot more to it than just that, which people tend to forget.
"Our job is to create a strategy and a series of big creative ideas that'll get more Londoners to see Time Out in this way, he added.
Earlier this year, Time Out underwent a redesign, which resulted in alterations to both the size and content of the magazine.
However, the revamp has failed to affect its circulations, which, according to this month's figures, are up by only 0.6 per cent period on period.
Time Out's publisher, Lesley Gill, said: "The redesign was a first step to consolidate our position. Now we're focused on growth and making the magazine central to London life. M&B's approach, based on developing big ideas that get people talking, is exactly what we need to achieve this."
M&B will be rolling out the new brand strategy across a host of media, working with an as-yet-unnamed media buying agency.
The win caps a busy summer for the agency, which secured the combined creative and strategic Xfm account in July, in a move away from Quiet Storm. M&B's first work for the brand is set to break in the next few weeks and will support the station's drivetime show fronted by Zoe Ball.
Last month, M&B was also appointed by Cadbury Trebor Bassett to help boost sales for its revamped Butterkist brand.