Mr. Forrest was arraigned at San Mateo County Superior Court in Redwood City and was expected to be released on $100,000 bail, according to Lynda Gledhill from the California attorney general's office.
A complaint filed by that office alleges Mr. Forrest possessed child porn on Dec. 20 and 21, 2012.
San Mateo County District Attorney Steve Wagstaffe, who said he has previously worked with Mr. Forrest in various professional capacities, turned the case over to Attorney General Kamala Harris.
At the arraignment, a judge outside of San Mateo County was brought in to further dismiss any concerns about the county's previous working relationship with Mr. Forrest, Mr. Wagstaffe said. "We want the public to feel comfortable," he said.
The complaint stems from a federal investigation by the U.S. Postal Inspector Service last December.
Mr. Forrest was placed on administrative leave on Dec. 21, 2012, after county officials announced the probation chief was under investigation. He announced his immediate retirement on Dec. 31.
Mr. Forrest had worked for the county's probation department since 1977 and was appointed chief in 2009. His salary in 2012 was $140,004.
Calvin "Cal" Remington is now serving as interim probation chief.
Mr. Wagstaffe said the U.S. Attorney General's Office turned the case over to the county, which in turn passed it along to the state.