Thom was born in Akron, Ohio in 1944. Her father was an engineer. He worked in the steel industry. Her mother worked from home as a homemaker. Thom listened to jazz and enjoyed Shakespeare. She credits these two interests at triggering her interest in activism. She graduated from Bryn Mawr College in 1966. While there, she became involved in the anti-war and civil rights movements. She was a fundraiser for the Student Nonviolent Coordinating Committee.  Thom became an editor at Ms. in 1972. She became executive editor in 1990. When she started working at Ms. she created an evaluation system about politicians. It was a feature in the magazine. She quit in 1992.

Thom lived and worked in New York City and never married. She enjoyed motorcycles. She was the editor-in-chief of the Women’s Media Center’s features department. She also co-wrote a book about Bella Abzug with Suzanne Braun Levine. She was killed in a motorcycle accident in Yonkers, New York on April 26, 2013. Thom had picked up her 1996 Honda Magna 750 from winter storage when she veered into traffic upon entering a highway. She struck a vehicle and was struck by another.