A member of the Los Angeles City Community Redevelopment Agency,[2] Rosenthal first won election to the California State Assembly in 1974, representing West Los Angeles.

[2] In 1982 he left the assembly to run for the state senate seat being vacated by Democrat Alan Sieroty and served in the upper house from 1982 until term-limits forced him from office in 1998.

[3] After the 1991 reapportionment moved Rosenthal's 22nd state senate district into the East Los Angeles area, he was basically left "districtless", although his current term didn't expire until 1994.

[4] He opted to run anyway in the newly drawn 23rd district, which picked up much of his old West Los Angeles territory but went on to lose the primary to then assemblyman Tom Hayden by less than 1000 votes[5] In 1994, he chose to seek his final term in the 20th district being vacated by term-limited state senate President Pro Tem David Roberti (D-Van Nuys).

The district, based largely in the San Fernando Valley, containted some territory Rosenthal had previously represented.