Born in Cumming, Georgia, Wellborn attended the common schools, Emory University, Oxford, Georgia, and the University of North Carolina at Chapel Hill.

He enlisted in the Confederate States Army in 1861 and served throughout the Civil War, attaining the rank of Captain in Company B, Fourth Georgia Cavalry.

At the close of the war, he settled in Atlanta. He studied law. He was admitted to the bar in 1866 and commenced the practice of law in Atlanta. In 1871, he moved to Dallas, Texas, and continued the practice of his profession.

Wellborn was elected as a Democratic Representative to the Forty-sixth and to the three succeeding Congresses (March 4, 1879 – March 3, 1887). Re-districting in 1882 changed the 6th District to the 3rd District. He served as chairman of the House Committee on Indian Affairs (Forty-eighth and Forty-ninth Congresses). He was an unsuccessful candidate for renomination in 1886 to the Fiftieth Congress.

Wellborn moved to San Diego, California, in 1887, and continued the practice of his profession for six years. He moved to Los Angeles, California, in 1893, where his daughter married Burton E. Green, a co-founder of Beverly Hills, California.

President Grover Cleveland appointed Wellborn as District Judge the United States District Court for the Southern District of California in 1895, which office he held until January 20, 1915, when he retired.

Olin Wellborn died at age 78 in Los Angeles. He is interred in Angelus-Rosedale Cemetery.