BETC has been awarded the account following two outdoor poster campaigns, one of which accused Mayor Boris Johnson of being a "pickpocket".
The appointment was chaired by Tessa Jowell, Labour MP for Dulwich and West Norwood, London.
BETC will work on a series of issue-driven ads for the Livingstone campaign in the lead-up to the London mayoral elections on 3 May.
Harry Barlow, communications adviser for Ken Livingstone, said: "BETC London showed us fantastic strategic thinking as well, as excellent creative ideas.
"This, combined with their awareness of the policy issues Ken is standing for, put them ahead of the curve. They are a new London office keen to prove themselves and we appreciate that enthusiasm."
The agency created a viral for the Livingstone campaign, called 'Poor Boris', showing the mayor riding his bicycle. The video claims Boris's mayoralty is a second job to supplement his work for The Daily Telegraph.
Johnson earns £250,000 a year for his columns at the newspaper, an amount he once described as "chicken feed".
BETC London has been "heavily involved" in Livingstone’s first party election broadcast, airing tomorrow (11 April).
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