 Thomason continued to practice law upon his moving to El Paso, in a law firm with Thomas Calloway Lea, Jr., and later J. G. McGrady and Eugene T. Edwards.He was a member of the Texas House of Representatives from 1917 to 1921, serving as Speaker in 1920 and 1921.Thomason served as the Mayor of El Paso from 1927 to 1930, and was elected to the 72nd United States Congress as a Democrat in 1930.