Justice Cotter graduated in 1972 with honors from Western Michigan University with a B.S. in Political Science and History, and graduated from Notre Dame Law School in 1977. After practicing law in her native South Bend, Indiana, for six years, she and her husband Michael moved to Montana, and in early 1984 began practicing law with John Hoyt in Great Falls. In 1985, they established the firm of Cotter & Cotter.
In 1992 and again in 1998, Justice Cotter received the Montana Trial Lawyers Association's Public Service Award for her contributions in the preparation and filing of amicus curiae briefs before the Montana Supreme Court. In 1993, she became chair of the Association's amicus committee, and served in that capacity until resigning to run for the Court in 1999.
From 1996 to 1998, Justice Cotter served as chair of Montana's lawyer representatives to the Ninth Circuit Judicial Conference. In 1998, she was chosen as one of five Ninth Circuit lawyers to serve on the Executive Committee for the Ninth Circuit Judicial Conference.
On November 7, 2000, Justice Cotter won election to the Montana Supreme Court and assumed the position previously held by retiring Justice William Hunt. She is currently serving an eight-year term on the Court. Justice Cotter is currently chair of the Supreme Court Commission, appointed to make recommendations regarding the adoption of a new Code of Judicial Conduct for the State of Montana.
Justice Cotter and her husband Michael have two children, Jack, age 18, and Kathleen, age 15.