Selden Leroy Osborne was born in Arizona in 1911, and raised in San Diego, California. Osborne graduated from Stanford University, where he became active in social reform and organizing; shortly after graduation he became a union organizer of agricultural workers in the San Joaquin and Imperial Valleys of California. He also became a member of the Socialist Party, becoming president of the local Palo Alto, California chapter at the age of twenty-one in 1932; in 1935 Osborne left the Socialist Party with the Trotskyites and formed the Social Worker's Party.

During World War II Osborne spent time in the army and worked as a merchant seaman, then worked on the waterfront in San Francisco as a probationary longshoreman in 1945. After the 1946 strike, he became a full time member of the International Longshoremen's and Warehousemen's Union (ILWU), Local 10. Throughout his time as a member of the union until his retirement in the 1970s, Osborne was heavily involved in attempts to reform the ILWU, and ending the leadership of the co-founder and president of the ILWU, Harry Bridges.

In the 1950s, Osborne became a pacifist and protested against nuclear armament during the Cold War. Osborne also joined the War Resister's League and completed the first coast-to-coast disarmament walk across the United States in 1976. Osborne died on February 5, 1994 in San Francisco, California.