Sebastian Ridley-Thomas, a Democrat, defeated former Culver City mayor and city clerk Christopher Armenta, 60 percent to 36 percent.
Ridley-Thomas, an aide to Los Angeles City Councilman Curren Price Jr., will fill the Assembly District 54 seat vacated this summer when then-Assemblywoman Holly Mitchell, a Democrat, resigned after she was elected to the state Senate.
Democrats comprise 63 percent of the district's registered voters. Just 11 percent are Republicans while 16 percent have no party preference. The district is entirely within Los Angeles County. In addition to Culver City, AD54 includes Century City, Baldwin Hills and the Los Angeles neighborhoods of Crenshaw and Westwood.
Ridley-Thomas was endorsed by Governor Jerry Brown and former L.A. Mayor Antonio Villaraigosa.
At 26, Sebastian Ridley-Thomas will be the youngest member of the Legislature when he is sworn in.
The Democratic Party already holds a supermajority in the California Assembly.
Election officials said that less than 7 percent of eligible voters cast ballots on Tuesday.
The Associated Press contributed to this report.
