He served in the military during the Civil War as a Sergeant in Company C. of the 187th Regiment of the Pennsylvania Volunteer Infantry. He enlisted as a Corporal on 7 Apr 1864. He was promoted to Full Sergeant on 6 Feb 1865. He was mustered out on 3 Aug 1865 in Harrisburg, Pennsylvania.

John S. Jenkins, of Pittston, spent his early life in Exeter, where he acquired the rudiments of an education in the public schools. In 1847 he started out for himself and found employment with Samuel Benedict, of Pittston; two years later, 1849, he began business for himself, boating coal on the North Branch canal, between Pittston and New York. He helped to take the first boat load of coal that the Pennsylvania Coal Company ever took to New York City. In the second year of the was of 1861-5 he enlisted as private, and was promoted sergeant of Company G, One Hundred and Eighty-seventh Pennsylvania Infantry, and served with the regiment until mustered out August 3, 1865. He then returned to Pittston and took charge of the Greenwood Colliery in August, 1865, and was its superintendent outside and inside till 1876. August 1, 1877, he was made superintendent of the Spring Brook railroad, a lumber carrying road chiefly, and from 1871 until 1876 he was interested in a large general store which was operated under the firm style of John S. Jenkins & Co. He then sold out, but in 1879 he started a new store in this same building and continued this with a slight interruption till 1882, then sold out. From 1879 to 1887 Mr. Jenkins was engaged in various mining enterprises, leasing and reletting coal mines and rights, in many cases developing coal lands which were supposed to be unproductive. In 1887 he purchased of John Jermyn a mining property in Blakeley borough, developed it into a paying operation, organized the Rush Brook Coal Company, and became its president, a position he held till 1899 when it was sold to the Ontario and Western Railroad. All that the company was, and all that the mine was, was due almost wholly to the efforts of John S. Jenkins. His efforts in business life have been successful and that success has been deserved.
Mr. Jenkins is a Republican, and has held the office of councilman of West Pittston a number of terms and is still serving. He is a Mason, a member of St. John's Lodge, No. 233, of Pittston; a member of the A. A. S. R., up to and including the thirty-second degree; of Keystone Consistory, S. P. R. S.; and also past commander of W. G. Nugent Post, No. 245, G. A. R. of Pittston.
Genealogical and Family History of the Wyoming and Lackawanna Valleys Pennsylvania, 1906