"No one is better equipped to repair America's alliances abroad and address the urgent needs of our communities at home," O'Malley said.
Clinton praised the governor for signing the nation's first statewide living wage law on Tuesday, a measure that requires state contractors to pay at least $8.50 to workers and $11.30 in parts of Maryland such as Baltimore and the Washington suburbs where it is more expensive to live.
Maryland is a strong Democratic state. Last year, O'Malley defeated Republican Robert Ehrlich, who was the first GOP governor of Maryland in 36 years. Clinton came to Maryland during that campaign to help raise money and build enthusiasm for O'Malley and other Maryland candidates.
Last month, Clinton named Maryland Sen. Barbara Mikulski the co-chair of her presidential bid. Mikulski's duties include speaking on Clinton's behalf and helping with campaign strategy.
Mikulski said at the time that she hoped to help Clinton break the "last public barrier for women in public life," the presidency.