Oxnard City Manager Ed Sotelo announced the selection Monday in a statement. It was not announced when Williams would begin working in Oxnard.
Williams, 44, will take over leadership of 237 police officers and 150 support staff. Oxnard is a city of approximately 200,000.
The Phoenix Police Dept. has 3,500 officers, 1,000 of whom Williams oversees in her division.
Williams is the second female police chief pick in Ventura County. LAPD veteran Kathleen Sheehan is head of Port Hueneme Police.
Williams is a 22-year police veteran.
The Ventura County Star reported a rift between the city of Oxnard's mayor and city council and Sotelo. The city council and mayor were reportedly not part of the selection process of Williams.
The Ventura County Star contributed to this report.