Villaraigosa maneuvered through reporters in the hallway before the event, flashing his trademark smile but ignoring shouted questions about reporter Mirthala Salinas.
When asked if he was trying to burnish his image by returning to city business, he replied: "Just doing my job."
Villaraigosa, 54, disappeared after confirming his relationship with Salinas on July 3. He had no public appearances on the Fourth of July and skipped the official opening of New York Sen. Hillary Rodham Clinton's Los Angeles headquarters even though he is a national co-chairman of her presidential campaign.
Salinas, a 35-year-old reporter for Telemundo's local affiliate, KVEA, once covered the mayor as a political reporter for the Spanish-language station. She was removed from the beat about 11 months ago.
The station placed her on leave Thursday while it investigates whether her relationship with Villaraigosa breached journalistic ethics.
Villaraigosa's wife, Corina, has filed for divorce. The couple have a 14-year-old daughter and an 18-year-old son. The mayor also has two adult daughters from previous relationships. When he and his wife wed in 1987, they merged their last names — his Villar and her Raigosa. The mayor will keep the Villaraigosa name, according to his office.