Hallinan grew up in a 22-room mansion in Ross, California. At age twelve Hallinan fell off his horse, fractured his skull, and spent five days stranded outside Yosemite before being rescued by helicopter.
As a young man Hallinan developed, in the words of California Supreme Court Justice Raymond Peters, a “habitual and continuing resort to fisticuffs to settle personal differences.” Hallinan became a ward of juvenile court in 1954 when he took a case of beer from three sailors after he and his brother had run them off the road on Point Reyes and beat them. The juvenile court banned Hallinan from Marin County so he had to get a job in a warehouse in Sacramento before clerking for the Longshoremen's Union in Hawaii. Shortly after turning eighteen Hallinan pleaded guilty to battery for punching the proprietor of a Lake County ski lodge. In 1957 Hallinan punched a fraternity brother who denied him admission to a private party. Hallinan was indicted in 1959 after he broke a man’s jaw during a brawl at a Greenbrae bowling alley. While at UC Berkeley Hallinan boxed for the Golden Bears and sparred with Cassius Clay in the 1960 Olympic boxing team eliminations. Due to a perceived skill at knockouts he went by the name “Kayo” Hallinan.
Hallinan’s propensity for fistfights continued in law school. When Hallinan and his brother were picketing in San Francisco against the House Un-American Activities Committee, some of their classmates arrived to picket the Hallinans. Strong words ensued and a fight was arranged in Golden Gate Park. Initially Hallinan was part of the crowd of UC Hastings student onlookers but he soon began a brawl with one of the opposing spectators. Hallinan is also known to have engaged in fistfights at a Young Democrats meeting and over a woman while at UC Hastings.
As a student Hallinan also became interested in nonviolent resistance. While attending the London School of Economics Hallinan was arrested with Lord Bertrand Russell during a Campaign for Nuclear Disarmament sit-down demonstration in front of the U.S. embassy. When he returned to America Hallinan joined the Student Non-Violent Coordinating Committee and spent the summer of 1963 in Mississippi. Local authorities jailed Hallinan twice but the U.S. Attorney General and the National Council of Churches intervened to secure his release.
Back in San Francisco Hallinan helped organize the W.E.B. DuBois Club to support Communist Party USA. In the fall of 1963 Hallinan became involved in the Ad Hoc Committee to End Discrimination and participated in sit-ins at the Sheraton Palace Hotel, Mel's Drive-In, and the Van Ness Avenue Cadillac showroom. Hallinan’s civil disobedience in the city resulted in six arrests and two separate criminal convictions.
After graduating UC Hastings, Hallinan’s criminal history proved an obstacle to his admission to the California State Bar. The Committee of Bar Examiners required several hearings over Hallninan’s moral character. Hallinan introduced evidence that his violent tendencies were the result of a thyroid deficiency. His mother, however, testified that Hallinan became violent in response to the bullying caused by his father’s vocal support of the Soviet Union during the Red Scare. California State Assemblymen Willie Brown and John L. Burton both testified that Hallinan possessed good moral character. The Committee questioned Hallinan about whether civil disobedience is compatible with being an attorney at law. Hallinan responded that he thought “it’s an unfortunate thing” that more German lawyers did not disobey the Third Reich.
The California State Bar refused to admit Hallinan. Hallinan appealed to the Supreme Court of California, and won. Justice Peters found that if the court denied professional licenses to everyone who engaged in a sit-in “we would deprive the community of the services of many highly qualified persons of the highest moral courage.” Justice Marshall F. McComb dissented, writing that Hallinan believes he has a “right to violate the law”.
Beginning his career during the peak of the 60's counterculture Hallinan defended hundreds of drug charges out of the Haight-Ashbury. Janis Joplin’s biographer alleges that Hallinan almost died after the singer shot him up with heroin at her Noe Street apartment. Hallinan successfully defended the Diggers after they were arrested for giving away free food on the steps of San Francisco City Hall. In 1968 he unsuccessfully defended the mutiny court-martial of the Presidio 27.
As an attorney, he successfully argued to have the murder convictions of serial killer Juan Corona overturned on appeal, and represented Corona in his retrial which resulted in 25 convictions for murder and a life sentence.
Hallinan unsuccessfully defended William Leonard Pickard for running an LSD lab in Mountain View, CA and latter used the DA office’s official letterhead to personally recommend Pickard be bailed after being caught in the largest LSD bust in history.
In 1975 Hallinan quickly left the Patty Hearst defense team after they rejected his involuntary intoxication theory of the case. Two years later, he represented 16-year-old Marlene Olive, who was accused (along with her adult boyfriend) of murdering her parents in the "barbecue murders" case.
In 1988, Hallinan left private practice for a political career, first serving for seven years on the San Francisco Board of Supervisors and then as district attorney. In 2004, he returned to private practice, focusing primarily on medical marijuana cases. In 2010, he defended Mitchell Brothers porn empire heir James Mitchell, who was accused of murdering his infant daughter's mother with a baseball bat. Mitchell fired Hallinan after seven months, and was then convicted.
In late 2014, Hallinan was temporarily suspended by the State Bar of California for commingling his and a trust client's funds.
Hallinan lost his first election campaign, for San Francisco District 5 Supervisor, to Harvey Milk in the 1977 San Francisco Board of Supervisors election. Hallinan ran again in 1988 and this time won a seat on the board.
In the 1995 elections the police union and both the city's daily newspapers endorsed recently fired senior prosecutor Bill Fazio against incumbent district attorney (DA) Arlo Smith. Hallinan successfully forced and won a runoff election for DA through a campaign run by his longtime aid Ross Mirkarimi. The former defense attorney promptly fired 14 senior prosecutors, leaving pink slips on their chairs during lunch, then posted an armed guard outside his new office in the Hall of Justice. Prominent Irish real estate developer Joe O'Donoghue confronted Hallinan about the firings while attending a birthday party at Izzy's Steaks and Chops. Hallinan responded by punching him. The resulting scuffle was lampooned by David Letterman.
When two prosecutors were caught having sex in their office Hallinan fired the man but retained the woman. Hallinan's chief assistant Marla Miller resigned over the scandal so Hallinan replaced her with David Millstein, the private attorney who had represented him in the sexual harassment lawsuit he settled out of court while a city supervisor.
In 1999 Hallinan was investigated for felony misappropriation of funds for a salary he paid out to his cousin. After 22-year veteran city Public Defender Jeff Brown, cousin of Jerry Brown, withdrew his endorsement of Hallinan, Governor Gray Davis appointed Jeff Brown to the California Public Utilities Commission. Mayor Willie Brown then appointed the daughter of State Senator John Burton to the open post, thirty-five years after the men had testified at Hallinan’s State Bar hearings.
After a close-fought reelection campaign in 1999, Hallinan's office sunk to the lowest case winning percentage of any DA’s office in the state. While serving as DA, he became a notable opponent of capital punishment. He also was a strong advocate on behalf of decriminalizing prostitution. In his tenure he supported medical marijuana and is now an advisor of NORML.
When Diane Whipple was mauled to death by a dog, Hallinan brought murder charges against its owners. After a nationally publicized trial that had to be moved to Los Angeles, Hallinan’s prosecutors, gay former Jesuit priest Jim Hammer and Kimberly Guilfoyle Newsom, won the conviction.
The entire SFPD leadership was indicted by Hallinan after a bar fight involving assistant police chief Alex Fagan's son. No police department in American history had been subject to such sweeping indictments and the scandal became known as Fajitagate. Most charges were latter dropped and the only two officers tried were acquitted. By indicting Chief Earl Sanders, the city's first black police chief who had helped win the lawsuit setting racial quotas on SFPD hiring, Hallinan was expected to lose crucial support from the city's black voters. Chief Sanders later sued Hallinan.
Hallinan was defeated in the next election for DA by Kamala Harris. When Harris took over in 2004, Hallinan returned to private practice with his son, focusing almost exclusively on medical marijuana cases.