Formed during a meeting on Thursday, April 25, 1861, by members of the "Scott Legion," a group of soldiers who had served under General Winfield Scott during the Mexican–American War, the 20th Pennsylvania Volunteer Infantry was led by William H. Gray, a 46-year-old resident of Philadelphia, who had been appointed as commanding officer of the regiment and awarded the rank of colonel at this same meeting, Gray then began his recruitment efforts the next day at multiple recruiting locations in Philadelphia.

[1][2][3] Mustering in at Philadelphia on April 30, the newly enlisted men then elected the following as Field and Staff Officers on May 7: George Moore, lieutenant colonel; Andrew H. Tippin, major; Edwin R. Biles, adjutant; Charles A. Jones, quartermaster; A.

The Captains and Lieutenants are all above thirty, or twenty-two years of age....Initially stationed at the Post Office in Philadelphia, the 20th Pennsylvania Volunteers were "clothed in the old United States blue jacket and pants," according to Bates, equipped with rifled muskets, and given basic training in both Scott's and Hardee's infantry tactics.

Honing their marching skills during a series of dress parades, they also engaged in periodic battalion drills at the State House yard before the regiment was moved to Suffolk Park, six miles outside of Philadelphia.

Stationed there for two weeks, during which the men received additional arms training, the regiment was then ordered to Chambersburg, Pennsylvania, where it was attached to Major General Robert Patterson's army as part of the 3rd Brigade, First Division.