She served with the Women's Army Corps and received a Bronze Star Medal for her service as a Corporal in the New Guinea campaign.

She began work in Japan at General MacArthur Headquarters as Chief of the Recruitment Section, Department of Army Civilians, Tokyo.

In 1950, the couple divorced, and Thorpe and her children left Japan and returned to the United States to live in Pearl River, New York, near her father's home.

[8] In late 1969 and 1970, Grace Thorpe joined a group of Native activists in their occupation of Alcatraz Island off the coast of San Francisco.

This occupation called national attention to a long history of Native grievances that the activists felt the federal government was too slow in addressing.

It made me put my furniture into storage and spend my life savings.”[9] During the occupation she managed publicity for the group and acted as a negotiator between the activists and the federal government.

After Alcatraz, Thorpe remained involved in agitation for Native rights, participating in further occupations of Fort Lawton, Washington in March 1970 and Nike Missile Base near Davis, California in November 1970.

She told a journalist in March 1971, “We Indian women decided to start beating the drum for ourselves… We want all Indian women who want to be active to join us in finding solutions to our problems.” The group's focus on family units derived in part from the history of the American government removing Native children from their families and sending them to live in white households or in boarding schools.

[14][10] Thorpe's environmental activism began in 1992 when she learned from a Daily Oklahoman article that her Sac and Fox tribe had accepted a federal grant to study the placement of radioactive waste on tribal land.

[15] The Sac and Fox, as well as sixteen other Native American tribes, accepted the grant, believing that the money would come without strings attached and would help alleviate their high unemployment.

She started working to convince her tribe to withdraw from the program, and went through the process outlined in the Sac and Fox constitution to reverse the decision of the elected tribal leaders; she gathered signatures of 50 tribe members on a petition calling for a special meeting, and at that meeting on February 29, 1992, 70 out of 75 members present voted to withdraw from the MRS program.

"[18] She helped found the National Environmental Coalition of Native Americans (NECONA) in 1993 and served as its president, traveling around the country and working to educate tribes about the dangers of storing nuclear waste and persuade them to refuse the MRS. Part of this work also involved persuading tribe leaders to rewrite their constitutions and revert their government to democracy rather than elected tribal councils, which were established under the Bureau of Indian Affairs in 1934 and were more susceptible to government pressure.