After the formation of the Confederacy and the Battle of Fort Sumter, Congress authorized a massive increase in the number of specialized engineer troops on August 3, 1861, to complement the growing Union Army.

It was on picket and guard duty in the vicinity of Fairfax seminary until August, when it was transferred to Franklin's brigade.

In September it transferred to Newton's brigade, and in November the original purpose of the organization was carried out and it was ordered to Alexandria to receive instruction in engineering.

[11] After the close of the campaign on the Peninsula, the 15th encamped at Harrison's landing and was then returned to Washington, joining the Army of the Potomac in the field November 17, 1862.

The remainder of the winter of 1862–63 was passed in camp at Falmouth, and during the Chancellorsville campaign the engineering brigade, of which the 15th formed a part, was instrumental in building bridges.

With the volunteer engineer brigade of the Army of the Potomac, it was present at Gettysburg, then joined in the southward movement of the army, shared in the Mine Run Campaign, and went into winter quarters near Brandy Station, VA.[7] Until January 1865, the 15th was engaged in siege duties before Petersburg, when three companies were detached and sent to North Carolina under Gen. Terry, where they were present at the fall of Fort Fisher, and in March, 1865, were sent to join the Army of the Ohio.

The remainder of the regiment remained at Petersburg until the final surrender, engaged in trench digging, mining, and other services incident to the siege.