Isaac was born on November 19, 1827 in Augusta, Georgia, United States, where his father was then in business, to Isaac Richards and Abigail Richardson (Munroe) St. John. He was a descendant of Matthias St. John who came to Dorchester, Massachussets, before 1632. A few years after the child's birth the family removed to New York City.
He entered Yale in 1841 and was graduated in 1845, the youngest member of his class. He began the study of law in New York City but gave up these studies to become assistant editor of the Baltimore Patriot, Baltimore, Maryland. In 1848, he gave up journalism to become a civil engineer.
Until 1855 St. John was on the engineering staff of the Baltimore & Ohio R. R. , and then moved to Georgia, where, for five years, he was in charge of construction divisions of the Blue Ridge R. R. When the Civil War began, St. John was in South Carolina, and he at once entered the Confederate service as a private of the Fort Hill guards of that state.
He was soon transferred, however, to Magruder's army of the peninsula for engineering duty, became chief engineer of that army, and, in February 1862, was commissioned captain of engineers. The energy and ability which he displayed in Magruder's army attracted the attention of the Confederate war department, and he was promoted to major on April 18, 1862, and assigned to duty in Richmond as chief of the nitre and mining bureau.
He efficiently performed the difficult task of supplying the Confederacy, which was blockaded on all sides, with nitre for the manufacture of gunpowder, and with metals for the construction of implements of war. His accomplishments were recognized by his successive promotions to lieutenant-colonel and colonel. Near the end of the war, when the problem of feeding the Confederate armies had become acute, St. John was selected to direct this important activity.
On February 16, 1865, he was appointed commissary-general with the rank of brigadier-general, and at once organized an efficient system for collecting and storing supplies and for forwarding them to the armies. He continued on this duty after the evacuation of Richmond and until the final collapse of the Confederacy. An article on the "Resources of the Confederacy" appeared under his name in the Southern Historical Society Papers, March 1877.
After the war he returned to his profession of civil engineering. From 1866 to 1869 he was chief engineer of the Louisville, Cincinnati, & Lexington R. R. and then for the next two years he was city engineer of Louisville, Kentucky.
In 1871 he declined reelection as city engineer and became consulting engineer of the Chesapeake & Ohio R. R.
He died suddenly of apoplexy while in residence at the "Greenbrier, " White Sulphur Springs, West Virfinia.
Isaac Munroe St. John sucessfully worked for the Baltimore and Ohio Railroad Company (mainly in Maryland) and the Blue Ridge Railroad Company in South Carolina before the Civil War. After the war, he worked for the Louisville, Cincinnati and Lexington Railroad in Kentucky; the city of Louisville, Kentucky; and the Chesapeake and Ohio Railway Company (mainly in Virginia and West Virginia). He was promoted to colonel on June 15, 1864. In 1873 he was chief engineer of the Elizabeth, Lexington & Big Sandy R. R. His name is included on the list of those participating in that meeting on the stone memorial in Washington, Georgia.
His wife was Ella J. Carrington, the daughter of Colonel J. L. Carrington of Richmond, Virginia. They were married on February 28, 1865, and had six children.
