Dentsu Aegis’ subsidiary, Carat, had held the account since 2002 when it picked up the consolidated business after a pitch against ZenithOptimedia and OMD UK. OMD had previously handled the bulk of the business.
MediaCom will work on the British Gas, Hive, Dyno and Local Heroes brands from 1 December, for at least three years.
The pitch process, which was run by ID Comms, has been running since March.
It is the biggest win for MediaCom since Josh Krichefski became chief executive in December last year. Earlier this year the agency lost E.On to Carat’s sister shop Vizeum.
Krichefski said: "We are absolutely thrilled to be starting this journey of transformation with such a significant household brand. We couldn't be happier."
Margaret Jobling, marketing director at British Gas, said: "We’re changing – moving from being just a utility company to being a modern, dynamic customer-centric organisation.
"We’re transforming our business, so that we continue to be relevant and future facing, and the pace of change in the media landscape is incredible."
Rick Hirst, Carat’s chief executive, said that the agency had a 100% retention rate this year up until now after it retained the Arla, Kellogg and Diageo accounts.
He said: "In the last year we've won £150m worth of new business and renewed relationships in statutory reviews. We had high hopes for this pitch, but you can't win them all."