Jocelyn attained the rank of brigadier general, and his command assignments included the 14th Infantry Regiment and the Department of the Columbia.

[11] The St. Johnsbury store was also the location of the local telegraph office, and a fellow employee taught Jocelyn telegraphy.

[14][b] In August 1863, Jocelyn decided to join the Union Army rather than accepting a relative's offer to finance his attendance at Dartmouth College.

[19] He was subsequently assigned to the encampment's quartermaster, and supervised several clerks who were responsible for providing uniforms, equipment, food, and pay to newly-arrived recruits.

[20] On the recommendation of Congressman Portus Baxter, in April 1864 he was considered for appointment as an officer; Jocelyn passed the examining board chaired by Colonel Silas Casey, and in August he was commissioned as a first lieutenant in the 115th United States Colored Infantry.

[16] After the war, Jocelyn served with his regiment in Indianola, Texas during occupation duty, and was mustered out of the United States Volunteers in February 1866.

[16][18] He continued to perform post-war occupation duty during the Reconstruction era, this time in South Carolina, where he commanded the regiment's Company C and served as its quartermaster and adjutant.

[18] In 1875, population increases that followed the development of gold mining that resulted from the 1867 Alaska Purchase led to the U.S. takeover of Fort Stikine, a former British trading post on the panhandle.

[16] During his time in Washington Territory, Jocelyn earned a reputation as an expert in military drill and ceremony, and units under his command won several competitions during the 1880s and 1890s.

[16] An experienced hunter, Jocelyn developed into an expert marksman; he won several of the army's individual shooting competitions during the 1880s and 1890s.

[27] In addition, his regimen for marksmanship training and practice enabled Company B, 21st Infantry to win several team shooting competitions.

[29] Among the lieutenants who carried out their initial assignments under Jocelyn's command in the 1880s and 1890s were Henry D. Styer and LaRoy S. Upton, both of whom served as brigadier generals in World War I.

[38] From 1904 to 1906, Jocelyn served as chief of staff of the army's Pacific Division, and in this assignment he aided in coordinating the US government's response to the 1906 San Francisco earthquake.

[18] In retirement, Jocelyn resided in Burlington, Vermont, where his interests included membership in the Military Order of the Loyal Legion of the United States.

[18] Jocelyn maintained an apartment in New York City and traveled extensively, including trips to Mexico, Bermuda, and Canada, in addition to fishing excursions in Florida.

[41] He was a frequent golfer, and played regularly for several years with Crosby P. Miller, a fellow Vermonter and retired brigadier general.