Kopp was elected to the San Francisco Board of Supervisors in 1971 and served until 1986, representing the West Portal neighborhood. In 1979, Kopp ran for mayor against Dianne Feinstein, but lost in a runoff election. This election also featured Jello Biafra. In 1986, Kopp ran for California State Senate as an independent in a heavily Democratic district straddling south San Francisco and northern San Mateo counties. Republican distaste for the Democratic nominee led them to financially support Kopp, who went on to win by one percentage point. He won reelection in 1990 and 1994. Term limits prevented Kopp from seeking reelection in 1998. In 1998, Republican then-Governor Pete Wilson appointed Kopp to a judgeship in San Mateo County. He served in that capacity until his retirement in 2004. During his time in the California State Senate, and afterward, Kopp, together with Mike Nevin, helped push through the BART extension to San Francisco International Airport with an airport station. In 1994, Kopp qualified an advisory ballot measure in San Francisco, Measure I, which advocated for a station inside the International Terminal. This resulted in the BART extension being built as a triangle, with the vertices being the San Bruno station at Tanforan Shopping Center, and not on the Caltrain Right-of-Way, Millbrae and SFO International Terminal. To get to all the stations on the extension, the BART train has to reverse at least once. The alternative rejected by Kopp was single station at San Bruno, California where the SFO People mover, BART and Caltrain would share a common station. The extension of the SFO People Mover across to the station was to be paid for as part of the traffic mitigation for the new International Terminal.
Kopp served as a member of the California High-Speed Rail Authority. As Chairman he worked to lead statewide efforts to develop an 800-mile high speed train network linking northern and southern California with fast, reliable, and environmentally friendly trains capable of traveling at up to. To help fund the project, Kopp led efforts to pass Proposition 1A in November 2008 - a $9.95 billion bond that has created the momentum that has led to the project receiving billions in federal funds. On 28 January 2009, without informing the CHSRA board, and without any vote, Kopp sent a list on Rail Authority letterhead to U.S. Senator Dianne Feinstein of "projects that CHSRA has identified...which can currently be commenced." The list omitted any requests for funds for two shovel ready projects: The Anaheim Station and Transbay Terminal extension in San Francisco, which would also serve as a key connection for transit riders arriving from the East Bay, following the decision to align the rail line through the South Bay and Pacheco Pass rather than the East Bay and Altamont Pass. Both the Anaheim Station and Transbay terminal projects had environmental review work completed based on input from the Authority. At the 5 March 2009 board meeting of the Rail Authority in Sacramento, when it became clear that another rail authority board member had obtained a copy of Kopp's letter, Kopp tried to hastily cure the record. He departed from the Board's meeting agenda and attempted to conduct an ad hoc vote of those board members in attendance regarding a project list which was never provided to the public and without any advance notice, a maneuver that the Train Riders Association of California considers a clear and egregious violation of the Brown Act.
Later life
in San Mateo County, a short, urban freeway connector, was renamed the "Quentin L. Kopp Freeway". The road was previously named the Portola Freeway by California's State Legislature, after Gaspar de Portolà. Kopp sent a letter to NPR correspondent Tamara Keith dated February 17, 2016 in which he described greetings exchanged between herself and fellow NPR host Audie Cornish as reminiscent of a "moronic schoolgirl" and that Cornish "debases a once respected profession with her flippant greeting." Keith posted two photos of the letter on her Twitter on March 8, 2016.
