Recruiting men from the counties of Bucks, Chester, Cumberland, Dauphin, Montgomery, and Union, as well as from the city of Philadelphia during June and July 1863, this regiment was composed of six companies of men who were commissioned for six months' service plus five companies of existing emergency militia units that had been assigned to picket and scout duties along the Susquehanna River and the roads leading toward Carlisle, Marysville and York during the Emergency of 1863 when regiments of the Confederate States Army invaded Pennsylvania.

[3] Ordered to Sir John's Run, West Virginia in early August 1863, the regiment was next assigned to guard part of the Baltimore and Ohio Railroad, as well as sections of the countryside near Winchester.

Assigned to detached service under Major Douglass, the remaining five companies were splot between duty stations at Philadelphia, Pottsville and Reading; they did not rejoin the rest of the regiment until the final muster-out.

[4] During an attack by Confederate cavalrymen on the Berkeley Springs command during the early part of September 1863, 20 members of the 20th Pennsylvania Cavalry were captured; horses and equipment were also lost.

After venturing nearly a hundred miles into Virginia, they engaged and defeated part of Imboden's command, capturing a number of small arms and several prisoners, and destroying a piece of artillery in the process.

[10] The 20th Pennsylvania Cavalry then became part of the Eighth Corps when General Crook succeeded Hunter as commander and, on July 18, lost an additional 14 men as it helped to defeat the forces of Early and Breckenridge at Snicker's Gap during the Battle of Cool Spring.

Engaged in "a daring charge down the Winchester Pike" on July 24, according to historian Samuel P. Bates, the regiment was then ordered by Crook to attack the rear of CSA General Jubal Early's army.

Separated from the bulk of Sheridan's command while it was engaged in the actions near Dinwiddie Court House, the 20th Pennsylvania "united in the attack which was made on the morning of the 1st of April, by which the enemy was driven by impetuous charges from one line of works after another, until he finally took shelter behind his main fortifications, on the White Oak Road," according to Bates.