The announcement comes just days before the California Republican Party's convention in Burlingame and is a setback to the Twin Peaks assemblyman's bid. Donnelly, reached Wednesday afternoon, said he was unaware of the departure.
"That's news to me," he said.
Kerns said in a prepared statement that she is "proud to have taken a candidate from the launch of a campaign as an unknown, underdog candidate to frontrunner status in the polls, as well as having cleared the field of our closest GOP competitor."
She said a filmmaker who produced campaign video ads for Donnelly has also left "making this now two top strategic advisers who have left."
Donnelly had made Kerns a significant part of his campaign, In January he posted an online video celebrating her promotion to campaign manager.
"They say beside every good leader is a great woman," Donnelly tells the camera after opening clips of Kerns running and Donnelly drinking coffee with his wife, Rowena. "I've got two."
Editor's note: This post was updated at 5:45 p.m. Wednesday to include the statement from Kerns.
PHOTO: Assemblyman Tim Donnelly, R-Twin Peaks, speaks in Baldwin Park Tuesday, Nov. 5, 2013. Associated Press/Nick Ut