The deal takes the fast-growing contractor’s tally of wins to £500m in this month alone.
Mears chairman Bob Holt said: “We have been working on behalf of Family Mosaic since 2004.
“This award represents the biggest contract award in Mears’ history.”
Holt added that the firm’s bid pipeline for contract tenders was in excess of £3 billion.
Family Mosaic, subject to the completion of the standstill period, have appointed Mears as preferred bidder.
The contract, which will run for 10 years, covers more than 20,000 homes in London and the Home Counties and is expected to commence in October 2010.
Mears will provide a single 24-hour call centre service for all tenants of Family Mosaic. It will also carry out responsive, void, gas maintenance, property surveying and estates management services.