Toschi was born to the Italian-American family of Sam and Millie Toschi in San Francisco, and was an alumnus of Galileo High School. Immediately upon graduation, he joined the US Army, and became a member of the 24th Infantry Division during the Korean War, honorably discharged in 1952. Returning to San Francisco, Toschi joined the San Francisco Police Department, where he served from 1952 to 1987. He was assigned to its homicide detail from 1966 to 1978. He is best known for his role as a chief investigator in the Zodiac Killer case, which he and his partner, Inspector Bill Armstrong, began to work on after the murder of taxi driver Paul Stine. He was also assigned to the Zebra murders team, and in 1985 received a meritorious conduct award for curtailing the career of a rapist/burglar. Toschi is known for his style of dress, including bow-ties, 'loud' plaid suits, bounteous curls, and an exaggerated trench-coat. In 1976, his quest for attention led him to send anonymous letters admiring his own efforts to Armistead Maupin, then a writer for the San Francisco Chronicle; revelation of that led to him being removed from the case in 1978. Toschi was also accused but later exonerated of writing one of the Zodiac letters, which the USPS crime lab verified as authentic but was later impeached by other experts; the suspicion ended Toschi's chance of replacing SFPD chief Charles Gain. Shortly after he left the SFPD, Toschi became Director of Security for St. Luke's Hospital in San Francisco's Mission District, and later served the same role for San Francisco's Pan Pacific Hotel. Toschi was vice president of North Star Security Services in Daly City. He was a technical advisor to the producers of the 2007 filmZodiac. Toschi married Carol Bacigalupi in 1957. They had three daughters.
Actor Mark Ruffalo portrayed Toschi in the David Fincher film Zodiac. Screenwriters Harry Julian Fink and R. M. Fink also modeled Harry Callahan, the main character of Dirty Harry portrayed by Clint Eastwood, on Toschi, while the film's villain - based on the Zodiac Killer - was called "The Scorpio Killer". Steve McQueen claimed that he copied Toschi's distinctive style of quick-draw shoulder-holster by wearing his gun upside down for the 1968 movieBullitt. McQueen also modeled much of his Bullitt character on Toschi.
