Williamson was elected as a Democrat to the United States Senate on November 4, 1930, to fill the vacancy caused by the resignation of Frederic M. Sackett and his initial replacement by John M. Robsion. He served only from December 1, 1930, to March 3, 1931, and was not a candidate for election to a full term.
Afterwards Williamson resumed the wholesale hardware business at Ashland, with residence in Catlettsburg, Kentucky, and was interested financially in various other business enterprises. He died in Cincinnati, Ohio, and was interred in the Ashland Cemetery Mausoleum.