He did not take the bar exam as he was admitted to the bar in 1905 under
diploma privilege. He began his law practice in
Oacoma, South Dakota in
Lyman County in that same year. He was a co-founder (along with his brother) of Murdo Coyote and the Prairie Sun. From 1905 to 1911 he was
prosecuting attorney of Lyman County. From 1911 until 1921 he served as
circuit court judge for the 11th judicial district. From March 1921 to March 1933 he was elected as a Republican to Congress (five succeeding terms). In 1912 he was a delegate to the
Republican National Convention, and served as chairman on the Committee on Expenditures in the
Department of the Interior for the 68th and 69th Congresses; and Committee on Expenditures in Executive Departments for the 70th and 71st Congresses. In 1932, following redistricting, Williamson ran for the reelection in
South Dakota's 2nd congressional district but was defeated by the Democrat
Theodore B. Werner, marking the end of his congressional tenure. ==Post political career==