Mills was first elected to the California State Assembly in 1960 to replace George G. Crawford, who had resigned the 79th District seat on February 15, 1960, to become a San Diego Municipal Court judge and George J. Lapthorne, who won the special election for the seat, but did not seek re-election later that year.

[3] Mills ran for California State Senate in 1966 and won the recently redistricted 40th District seat.

In arguing for the bill, which was later approved by voters, Mills said that a full-time legislature would be able to respond more quickly to the needs of a fast-growing state.

[2] During his time in the legislature, Mills was also a champion for the preservation of historic buildings and for public transportation authoring several laws that would create lasting changes in the state of California.

[2] In 1972, he authored a state law that would come to be known as the Mills Act, which allowed cities to reduce property tax for owners of a historic building in exchange for its continued preservation.

[4] Mills was inspired to write the law when a developer proposed demolishing the historic Hotel del Coronado in his district.

[2][4] Mills was the curator of the San Diego History Center's Junípero Serra Museum, from 1955 to 1960, when he left to run for the state assembly.