Cooke received his country's highest award for bravery during combat, the Medal of Honor, for his actions at the First Battle of Bull Run in Virginia on 21 July 1861.

[3] When the American Civil War began, Cooke organized Company K of the 4th Pennsylvania Infantry for three months' service with the Union Army in April 1861, in which he served as captain.

The regiment's enlistment expired on the day before the First Battle of Bull Run began on 21 July and its men insisted on returning to Pennsylvania rather than staying to fight.

Cooke, after finding that only a half dozen of his men stepped forward to fight in response to his appeal for them to stay, left them in disgust and initially started for the camp of the New York Fire Zouaves to serve as a private before being told he could be more useful with the staff of a unit.

He also served as a trustee of the Bringhurst Fund, which was established to provide affordable housing for the poor,[10] and was a vestryman of St. John's Protestant Episcopal Church.