[1] He entered the army corps of engineers as a second lieutenant upon his graduation from the United States Military Academy at West Point, New York, on July 1, 1839.

[1] He ranked fourth in a graduating class[1] that included future fellow Civil War generals Halleck, Canby, Hunt, and Ord.

Gilmer served in the Mexican War as Chief Engineer of the Army of the West in the New Mexico Territory and helped design and construct Fort Marcy in Santa Fe.

Until 1861, he was active in making surveys, constructing fortifications in various locations including San Francisco, California, and executing various river and harbor improvements.

These elaborate defenses would prove difficult to seize in frontal assaults, forcing the Union army to lay siege to Atlanta in the summer of 1864.