DDB won the business after a four-way pitch, managed by COI Communications, against Soul, WCRS and Miles Calcraft Briginshaw Duffy. ZenithOptimedia will handle media planning and buying.
Andrew Nye, the head of communications at London Elects, the independent body responsible for managing the 2004 elections, said: "We appointed DDB after a particularly competitive pitch. It demonstrated clear thinking and strong creative ideas that are in line with our remit to explain how London's elections on the 10 June work."
The appointment comes a day after the mayor, Ken Livingstone, was confirmed as Labour's London candidate.
DDB London will create a campaign to highlight the elections, tell people how and when to vote and direct voters toward further information. It is likely to include press, radio and outdoor. All advertising will include the web address www.londonelects.org.uk and the helpline number 0800 3280 280.
Thirty-four per cent of London's electorate voted in the mayoral and London Assembly elections in 2000, above the national average for local government elections.
The London Elects advertising will run in addition to UK-wide activity planned by the Electoral Commission and developed by St Luke's.