John Coates enlisted in the Union Army on September 4, 1861. He was commissioned as a Captain with Company I, New Jersey 10th Infantry Regiment on November 26, 1861.

William Bryan of Beverly, New Jersey recruited and organized this regiment which was known as the "Olden Legion". John Y. Foster, whose history of New Jersey' Civil War units written in 1868, states that "On the first roster of the regiment, after being placed in State service, is this endorsement: "This regiment was raised by individuals, not authorized by the State, and accepted by the War Department as an independent organization, some time in the fall of 1861, and was not known by the State authorities until it was placed under their care, January 29, 1862."

When the organization of the regiment was completed with nine companies of infantry and one company of cavalry, it was established at Camp Beverly, New Jersey where William Bryan lived. The unit proceeded to Washington, D.C., on December 26, 1861, with 35 officers, 883 non-commissioned officers and privates, for a total of 918 men.

After they marched to Camp Clay on the Bladensburg Turnpike, a location approximately one mile from Washington, they were reorganized and designated the 10th New Jersey Infantry. Soon after being reorganized the cavalry company, Company D, was discharged and a new company was raised that April. In fact, the regiment was not very effective by February 1862 when many of the cavalry company were under arrest for refusing to do infantry duties

Captain Coates mustered out on March 6, 1862.

John Coates was a member of the G.A.R., belonging to Thomas M.K. Lee Post No. 5 of the G.A.R., where his son John W. Coates was also a member, having served for over 3 years as a Private in Company B, 3rd New Jersey Infantry.

Camden's 1863 City Directory shows John Coates at 124 West Street, where he was still living in November of 1899. The Census of 1870 shows John Coates living with his wife and three children. John W., 30, Lewis C., 24, and Frances, 18. The 1880 Census shows John Coates, his wife Rebecca and sons John W. and Lewis C. Coates living at 124 West Street. Daughter Frances had married Dr. Onan B. Gross, a prominent Camden physician. John W. Coates worked with his father as a bricklayer, while Lewis was a grocer.

In February of 1899 John Coates son John W. Coates died suddenly while visiting his sister Frances and her husband, Dr. Onan B. Gross, at their home at 700 Market Street. Still living at 124 West Street and conducting his real estate business in November of 1899, John Coates reported a robbery in his home. This proved not to have been the case. John Coates moved in with his daughter and son-in-law shortly thereafter.

John Coates passed away in January of 1903. When his estate was finally settled in January of 1905, it was valued at 176,485.18

John Coates involvement in real estate and investments in Camden kept his name alive in the city long after his passing. Three small streets bore the name of Coates. As of 2014 one, Coates Court, is completely obliterated and there is very little physical evidence too show the other two, Coates Avenue and Coates Alley, ever existed.

Son Louis C. Coates was still living in Camden as late as 1925.