Effective March 8, 1865, Williston promoted to captain. He was brevetted captain on May 3, 1863, for "gallant and meritorious services in action at Salem Heights, Virginia" and brevetted Major for the same at the Battle of Gettysburg on July 3, 1863. He was brevetted Lieutenant Colonel on September 19, 1864 in the Battle of Winchester and brevetted Colonel on March 13, 1865.