After graduation Charles G. Williams was admitted to the bar and commenced practice in Janesville. He served in the Wisconsin State Senate 1869-1872 and was twice chosen president pro tempore of that body.
Williams was elected as a Republican to the Forty-third and to the four succeeding Congresses (March 4, 1873-March 3, 1883). He represented Wisconsin's 1st congressional district. He served as chairman of the Committee on Foreign Affairs during the Forty-seventh Congress. He was an unsuccessful candidate for reelection in 1882 to the Forty-eighth Congress. He resumed the practice of law.