The agency defended the business in a pitch against Archibald Ingall Stretton, Craik Jones Watson Mitchell Voelkel and Carlson. The shortlist was drawn up by the intermediary Creative Brief.
Geronimo has worked on the Royal Bank of Scotland-owned brand since 2003, when it was appointed to its pet insurance account.
Tullo Marshall Warren then handled the remainder of the business, which was consolidated into Geronimo in 2004.
The direct marketing review was called in June. It followed the insurer's decision to move its financial services advertising account from Clemmow Hornby Inge into M&C Saatchi in March after less than a year.
M&C Saatchi, which also handles advertising for the Royal Bank of Scotland-owned NatWest Bank, already held Direct Line's insurance business. Direct marketing for the brand's financial services products, such as loans, mortgages and savings accounts, is handled by Hall Moore CHI.
Both reviews were overseen by Mike Tildesley, who joined Direct Line as its marketing director in August last year. He was previously the marketing director at the Royal & SunAlliance insurance band More Th>n.
According to figures from Nielsen Media Research, Direct Line spent £5.6 million on direct mail alone in the past 12 months. Its total expenditure across all media was £38.7 million.