[1][2] When the American Civil War began in 1861 McGlashan joined the Confederate States Army as a private in the 29th Georgia Infantry Regiment and quickly was promoted to sergeant.

Afterwards they joined the Army of Northern Virginia and became a part of Gen. James Longstreet's First Corps – an assignment the regiment would keep for the duration of the war.

[4] At the Battle of Gettysburg the regiment, having 302 men present for duty, was in the brigade of Paul J. Semmes in the division of Lafayette McLaws.

[4][6][7] When the regimental commander, Col. William R. Manning, received a medical discharge on July 31 McGlashan was chosen to be his successor and was promoted to colonel.

The brigade, now under command of Goode Bryan, was too late for the Battle of Chickamauga but in time for the ensuing campaigns of Chattanooga and Knoxville.

[8] In January and February 1865 McGlashan commanded the brigade in Simms's absence, but returned to his regiment for the Appomattox Campaign.

Some days earlier President Jefferson Davis, priorly to fleeing from Richmond, had signed McGlashan's promotion to brigadier general; but it was not delivered or confirmed by Congress in time.

He was responsible for the forming of the fire department, which he utilised to keep up order while the city was under martial law and during the early reconstruction.

Peter McGlashan died on a trip to the Isle of Hope near Savannah when he drowned close to the beach on June 13, 1908.