During her time in the United States, Kelly was active in the U.S. civil rights and anti–Vietnam War movements and worked as a volunteer for the 1968 presidential campaigns of first Robert Kennedy and then Hubert Humphrey. In 1971 she received an MA in political science from the University of Amsterdam. Two years later, she joined the administrative staff of the European Community in Brussels. Beginning in the mid-1970s, she became increasingly engaged in environmental, peace, and feminist activities in West Germany.
In 1979, Kelly left the Social Democratic Party (SDP) and became one of the founding members of the Green Party. Elected to the German Bundestag as a Green member in 1983, Kelly became a staunch proponent of environmental policies and vehemently objected to the placement of U.S. nuclear missiles in West Germany. In 1982 she was awarded the annual alternative Nobel Peace Prize.
Kelly had a reputation as a smart and sensitive activist who alternatively confused and dazzled the West German public with her idealism and charisma. She was a prominent fixture in the Green Party and a much-sought-after speaker at peace and antinuclear rallies. Despite her uncompromising stance on matters of disarmament, the environment, and nonviolent protest, she refrained from endorsing her party's anti-American and prosocialist leanings.
In the late 1980s, Kelly lost influence with the Greens. In the wake of German reunification, the Greens lost all their parliamentary seats in the 1990 elections, and Kelly became estranged from her political colleagues. After the defeat, she largely withdrew from public life. On 1 October 1992, Kelly's partner, former Green Party deputy Gert Bastian, allegedly shot her to death as she slept in their Bonn home. Bastian then took his own life.
Kelly, Petra. Thinking Green! Essays on Environmentalism, Feminism, and Nonviolence. Berkeley, CA: Parallax, 1994.; Parkin, Sara. The Life and Death of Petra Kelly. New York: HarperCollins, 1994.