Fonda and Tom Hayden, her then-husband and fellow political activist, sent Thatcher a telegram, the Belfast Telegraph reported. At the time, Irish republican prisoners held in Northern Ireland were on hunger strike, demanding to be treated as political prisoners and not as criminals.
"Please save the lives of the hunger strikers by granting them political status. The sanctity of human life must override any political considerations," their message said.
Fonda was an outspoken opponent of the Vietnam War, angering U.S. conservatives who nicknamed her "Hanoi Jane" after she visited North Vietnam. She and Hayden took up many other causes.
Thatcher was deluged with letters from around the world, many of them urging her to grant the prisoners' demands, something she never did.
The prisoners gave up the hunger strike after 10 men died.