Williamson was licensed to preach by the Presbyterian Church of Minnesota in 1859, and in the year following his graduation he worked as a pastor at Allensville and Zoar, Indiana.

He arrived at Crow Creek Indian Reservation on May 31, 1863, which at that time was governed by Clark W. Thompson as superintendent and had a population of 1,300 Indigenous people.

[3] He was appointed United States special agent for the Flandreau Indian Reservation in 1873 and remained in office for five years.

[8] His publications include: Williamson married Sarah A. Vannice (born 1843) on April 27, 1866.

[8] Williamson died in Greenwood, Charles Mix County, South Dakota,[9] on October 3, 1917, from pneumonia.