Sellins was born Fannie Mooney in New Orleans in 1872. She married a garment worker, Charles Sellins, in St. Louis. After his death, she took a job in a garment shop to support herself and her four children. Eventually she moved from St. Louis to Chicago and soon was involved with the union movement there. Later, because of her outstanding abilities, she became an organizer for the United Mine Workers of America (UMWA). She was sent to work in the nonunion coal fields of West Virginia. While there, she was arrested for "inciting to riot" and was sent to prison. She served six months of her sentence before she was pardoned by President Woodrow Wilson.

In 1917 Sellins moved to New Kensington, PA, to work under labor leader Philip Murray as an organizer and troubleshooter for UMWA District 5. She quickly became involved in the union´s efforts to organize miners in the Allegheny Valley, a notoriously anti-union area. Due largely to Sellins efforts, many thousands of miners and other workers in this district were organized. She was fatally shot in the back by three mine guards in Pennsylvania while picketing on August 26, 1919.