[1][2] Pecson was born in Barrio Libsong in Lingayen, Pangasinan as the second child of Victor Tomelden, a census inspector, and Maria Paz Palisoc, a teacher.

She eventually joined the faculty of the Zamboanga Normal School (now Western Mindanao State University) when her husband was assigned to teach there.

She married Potenciano Pecson, a teacher and lawyer, who was elected as representative for the First District of Pangasinan from 1928 to 1935 and was later appointed an associate justice of the Court of Appeals in 1953.

Apart from being a member of Commission on Appointments and of the Senate Electoral Tribunal, Pecson pioneered Philippines laws that included the 1953 Free and Compulsory Education Act, the Vocational Education Act, laws related to establishing training facilities for instructors of arts and trades in certain national schools, and laws that upgraded the School of Forestry of the University of the Philippines and the Philippine Normal School into colleges.

The main building of the Philippine Normal University in Manila was renamed Geronima T. Pecson Hall in 2005 to honor her.