"She knows how to make government work for ordinary people and she's been doing it for decades, not only when times are good, but especially during periods of crisis," Clinton said in a letter.
The mayoral candidate visited the California State University, Northridge, campus and spoke about the endorsement of the former president.
"President Clinton has been a role model for me," Greuel said. "He has shown what it means to be a leader."
Greuel worked for the U.S. Department of Housing and Urban Development in Clinton's administration. She was part of the team that Clinton assigned to Los Angeles in 1994 as a field officer to help coordinate the federal government's response to the Northridge earthquake.
Greuel was an early supporter of Hillary Clinton when she ran for president in 2008. Greuel's mayoral opponent, Eric Garcetti, backed Barack Obama.
In a statement Monday, the Garcetti campaign responded by saying, "Bill Clinton has been a constant refrain in Greuel's campaign, so this is no surprise. In fact, the DWP union has used Clinton in over $1 million worth of TV ads for Greuel in the primary."
In addition to Clinton, Greuel has endorsements from the Los Angeles County Federation of Labor, former Mayor Richard Riordan and county Supervisor Mark Ridley-Thomas.
Garcetti received endorsements from council members Paul Koretz, Joe Buscaino and Paul Krekorian.
The Clinton endorsement comes as Greuel faces sudden changes to her campaign team. Four key campaign members have resigned and Greuel replaced her campaign manager with Janelle Erickson, who is a veteran staffer of Mayor Antonio Villaraigosa. Greuel said the changes don't indicate any problems with her campaign.
"I think this is a natural," she said. "What happens in campaigns, you go from one step to the next, and we are excited about the direction we are going in."