Lewis was born in Clarendon, New York, and in 1845 he settled in Columbus, Wisconsin. He held the positions of district attorney and county judge, and he was elected to terms in both the Wisconsin State Assembly and the State Senate. He also served as the fourth Lieutenant Governor of Wisconsin from 1854 to 1856, Wisconsin secretary of state from 1862 to 1864, and governor from 1864 to 1866, making him the last Wisconsin governor to hold the office during the Civil War. His former home, now known as the Gov. James T. Lewis House, is listed on the National Register of Historic Places.