Martin Maginnis was born in western New York on October 27, 1841, the son of Irish immigrant parents. He later moved with his parents to a farm in Minnesota. After finishing school, Maginnis started work as a newspaperman on the Redwing Sentinel.

In response to President Lincoln's first call for volunteers, Maginnis joined the First Minnesota Infantry Regiment as a sergeant. He served with his regiment in the battles of First and Second Bull Run, Antietam, Fredericksburg, Chancellorsville, Gettysburg, and others. By the time of his discharge at the end of the war, he had risen to the rank of major.

After a brief return to his newspaper career in Minnesota, Maginnis joined the gold rush, arriving in Helena, Montana Territory, in September 1866. Finding little success as a gold prospector, he resumed his journalism career. In partnership with others, he published the Rocky Mountain Gazette, which later became the Helena Independent. In 1868 he married Louise E. Mann of Pontiac, Michigan. Louise was born in 1847 to Irish parents and was a benedictine.

In 1872, Maginnis, a Democrat, was elected Montana's territorial delegate to Congress. As a non-voting delegate, he promoted military expansion, reduction of the size of Montana's Indian reservations, railroad building, and land and timber laws to encourage settlement of Montana. After his retirement from Congress in 1885, Maginnis continued his political activity as a delegate to the 1889 Constitutional Convention and as an unsuccessful candidate for Montana's first U.S. Senator. He and William A. Clark were defeated by Republicans Wilbur F. Sanders and Thomas C. Power. Maginnis later served as U.S. Mineral Land Commissioner. In this position, he was successful in preventing the railroad land grants from including mineral rights. These rights were reserved by the federal government for individual developers.

Martin Maginnis died in 1919 in California.