[5] Platt responded to President Abraham Lincoln's call for volunteers following the fall of Fort Sumter to Confederate States Army troops in mid-April 1861.

After enlisting for a three-year term of military service in Philadelphia on August 5 of that year, Platt officially mustered in that same day as a private with Troop H of the 6th U.S.

Stationed at Belle Plain from November 24 until December 12, they then marched toward Fredericksburg, engaged briefly with the enemy, and made camp near Falmouth the following day.

[9][10][11] While fighting with his regiment during day three of the Battle of Gettysburg (July 3, 1863), Platt performed the act of valor for which he would later be awarded the U.S. Medal of Honor.

[9] Platt and other members of the 6th U.S. Cavalry were then detailed to service as the body guard for Union Major-General Philip H. Sheridan,[15][9] a duty he performed only briefly.

With his initial three-year term of service set to expire before the end of the summer, Platt opted to muster out honorably on August 5.

[16] Following his honorable discharge from the military in August 1864, Platt returned home to Philadelphia, where he wed Pennsylvania native Eliza Kelly.