With the outbreak of the Civil War in early 1861, Gillmore was assigned to the staff of Brig.
Gen.
Thomas W. Sherman and accompanied him to Port Royal, South Carolina.
After being appointed as a brigadier general, Gillmore took charge of the siege operations against Fort Pulaski.
A staunch advocate of the relatively new naval rifled guns, he was the first officer to effectively use them to knock out an enemy stone fortification.
More than 5,000 artillery shells fell on Pulaski from a range of 1,700 yards during the short siege, which resulted in the fort's surrender after its walls were breached.

The result of the efforts to breach a fort of such strength and at such a distance confers high honor on the engineering skill and self-reliant capacity of General Gilmore.
Failure in an attempt made in opposition to the opinion of the ablest engineers in the army would have destroyed him.
Success, which in this case is wholly attributable to his talent, energy, and independence, deserves a corresponding reward. —
New York Tribune
Although he was one of the best artillerists and engineers in the army he was not well respected by his men.
With the outbreak of the Civil War in early 1861, Gillmore was assigned to the staff of Brig. Gen.Thomas W. Sherman and accompanied him to Port Royal, South Carolina. After being appointed as a brigadier general, Gillmore took charge of the siege operations against Fort Pulaski. A staunch advocate of the relatively new naval rifled guns, he was the first officer to effectively use them to knock out an enemy stone fortification. More than 5,000 artillery shells fell on Pulaski from a range of 1,700 yards during the short siege, which resulted in the fort's surrender after its walls were breached.
The result of the efforts to breach a fort of such strength and at such a distance confers high honor on the engineering skill and self-reliant capacity of General Gilmore. Failure in an attempt made in opposition to the opinion of the ablest engineers in the army would have destroyed him. Success, which in this case is wholly attributable to his talent, energy, and independence, deserves a corresponding reward. —New York Tribune
Although he was one of the best artillerists and engineers in the army he was not well respected by his men.
Quincy Adams Gillmore