A staff aide to Sacramento Mayor Kevin Johnson has resigned amid allegations that she used her city-issued credit card to make thousands of dollars in personal purchases, including a trip to Disneyland, The Bee has learned.

Lisa Serna-Mayorga resigned Thursday following a monthlong inquiry into purchases on her credit card, according to three sources with knowledge of the situation who were not authorized to speak about the matter because it relates to personnel issues.

Johnson spokesman Joaquin McPeek would not discuss details of Serna-Mayorga's departure, but did say the mayor "is saddened and disappointed."

"These actions are unacceptable," McPeek said. "We are working closely with the city to address them."

As a result of the incident, City Manager John Shirey has directed his staff to conduct an audit of Serna-Mayorga's city credit card usage for the past three years, said city spokeswoman Amy Williams. The mayor requested that audit, Williams said.

Serna-Mayorga, 39, comes from a well-known Sacramento political family. Her father was former Mayor Joe Serna Jr., who died in 1999. Her brother is current Sacramento County Supervisor Phil Serna. When Johnson first ran for mayor in 2008, Serna-Mayorga was one of his most vocal and visible supporters.

One of Johnson's first actions as mayor was to hire Serna-Mayorga as the City Council operations manager, a job that includes overseeing the expenses and many other daily tasks of the council and mayoral offices. As council operations manager, she made $74,000 a year, according to the state controller's office.

Two telephone numbers listed for Serna-Mayorga were not accepting messages on Monday. She did not return an emailed request for comment, and her Facebook page has been deactivated.

According to the sources, a year-end review of Serna-Mayorga's expenses for fiscal year 2011-12 showed that she had made personal purchases with her city credit card. That discovery came a few days after Serna-Mayorga had been moved from her post as City Council operations manager to director of constituent affairs in the mayor's office.

Serna-Mayorga's credit card privileges were immediately revoked by the mayor's office and she reimbursed the city $3,000, the sources said.

At first, the sources said, city officials thought the personal purchases – which included the Disney trip, gasoline and meals – were limited to the 2011-12 fiscal year.

However, a more detailed inquiry revealed other personal purchases going back at least another year, they said. That information was relayed to the mayor's office last Tuesday. On Wednesday, Serna-Mayorga wrote the city a check for an additional $6,000, according to the sources.

Serna-Mayorga resigned Thursday.

Serna-Mayorga has been a close Johnson ally for years. She has long been active in St. HOPE Public Schools, a nonprofit organization founded by Johnson that operates charter schools in Oak Park, and the group's website lists her as the 2011-12 board secretary.

Last month, Serna-Mayorga was moved from the council operations job after the City Council stripped the mayor of his authority to fill the post. The shift was spearheaded by Councilwoman Sandy Sheedy, a frequent opponent of the mayor. Johnson criticized the council action as "petty political game-playing."

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