[8][9] Weinberg's first participation in a political organization occurred in 1963, when he joined the Berkeley chapter of CORE (Congress of Racial Equality).

[2][5] He returned to Berkeley and began his second semester of grad school in the fall of 1963 but then withdrew mid-semester to devote himself full-time to civil rights activities.

He refused to show his identification to the campus police and was arrested at noon[12] for violating the university's new rules regarding student political activism.

[16] After being confined in the police car for 32 hours,[17][18] Weinberg was then booked and freed as the agreement stipulated that the university would not press charges against him.

[20] Jack Weinberg called his close friend, classmate and free speech supporter, John Wingfield McGuire (also majoring in mathematics) to pick him up from jail and bring him to John and Rosemary McGuire's house on Russell Street near Telegraph Avenue, where Jack began making phone calls to begin organizing the next critical steps in the free speech movement.

The first meeting of FSM (Free Speech Movement) took place on October 3, in Art Goldberg's apartment.

[24] An Oakland Tribune photo from early January 1965 shows Weinberg speaking, alongside Savio, to a large campus crowd.

Often misattributed to Abbie Hoffman, Jerry Rubin, the Beatles, and others, Weinberg coined the phrase during a November 1964 interview about the Free Speech Movement with a reporter for the San Francisco Chronicle.

That annoyed Weinberg, who has said: I've done some things in my life I think are very important, and my one sentence in history turns out to be something I said off the top of my head which became completely distorted and misunderstood.

[31][32][33] On Friday night, October 15, 1965, the VDC held an anti-war march that began at the UC Berkeley campus and was intended to end at the Oakland Army Terminal.

[36] At the head of the march was a banner carried by a line of marchers, then a sound truck containing VDC leaders including Jack Weinberg, Bettina Aptheker, Jerry Rubin, Stephen Smale, Steve Weissman, Frank Bardacke, and Robert Scheer.

[4] Weinberg has said that the Stop the Draft Week protests of October 16–21, 1967, were the first clear demonstration that the radical part of the Anti-Vietnam war movement was coming up against its own limitations.

[41] In November 1968, Weinberg was the Peace and Freedom Party candidate for congress in California's 26th congressional district election (Los Angeles area);[42] he received 3% of the vote.

In 1973, he was a participant in wildcat strikes at Chrysler plants in Detroit, Michigan, as a member of UAW (United Automobile Workers) Local 212.

[47] In 1982, Weinberg led a coalition of environmentalists, unionists, and community members in defeating a proposal to construct a nuclear power plant in Indiana on Lake Michigan.