Hayden died in his home in Santa Monica “after a lengthy illness”, the Los Angeles Times reports.
He was a member of the “Chicago seven” charged with conspiracy over anti-Vietnam war protests in 1968 and eventually acquitted.
Hayden later served in the California state assembly and Senate for nearly two decades. He was married to actress Jane Fonda between 1973 and 1990.
Born in Michigan in 1939, he became an activist during his time at the University of Michigan, where he helped to found Students for a Democratic Society (SDS).
While there, he wrote a policy document called the Port Huron Statement, which he styled the “agenda for a generation”.
Mr Hayden and the SDS went on to become a major influence on the 1960s protest movement, particularly against the Vietnam war.
“Rarely, if ever, in American history has a generation begun with higher ideals and experienced greater trauma than those who lived fully the short time from 1960 to 1968,” he wrote in the essay Streets of Chicago.
In 1968, Mr Hayden was part of a controversial anti-war demonstration in Chicago, timed to coincide with the Democratic National Convention.
The protest turned violent, with eight people – including Mr Hayden – charged with conspiracy and crossing state lines to incite a riot.
The so-called Chicago seven trial – originally the Chicago eight, before one defendant was tried separately – ran for years, with appeals and retrials. Mr Hayden was eventually cleared of all charges.
In 1973, he married actress Jane Fonda, who was herself an outspoken critic of the Vietnam War. She was internationally famous and wealthy, while he was still seen in some quarters as an anti-establishment troublemaker.
He would go on to reinvent himself in the coming decades, moving away from the image of a long-haired student protester.
He turned his attention to mainstream politics in the late 1970s, earning himself a place in the California State Assembly in 1982. A decade later, shortly after his divorce from Fonda, he moved on to the California Senate.
He also became a prolific writer of books and essays, and served as a columnist for several outlets.
Fifty years after he wrote the Port Huron statement, about a generation “looking uncomfortably to the world we inherit”, he wrote that the concentration of wealth in the hands of the elite was a “mountain untouched” .
Writing in The Guardian in 2012, he called the Occupy Wall Street protests a “new force in the world”.
“The Occupy movement, and kindred spirits from the Middle East to China, is driven by young people who feel unrepresented by the institutions, disenfranchised economically, and threatened by an environmental catastrophe,” he said.
“The direct action movement of the early 1960s was similar in nature.”
Hayden married actress Barbara Williams in 1993, and had a son, Liam.
Join GhanaStar.com to receive daily email alerts of breaking news in Ghana. GhanaStar.com is your source for all Ghana News. Get the latest Ghana news, breaking news, sports, politics, entertainment and more about Ghana, Africa and beyond.