He was raised and educated in the public schools of his community with his siblings: Nancy (1831–1884), Andrew (1833–1863), Sarah Ann (1834–1894), Thomas Sharp Jr. (1838–1898), Alexander H. (1840–1913), Rebecca A.

Just 15 years old at the time, he then officially mustered in for duty as a drummer boy with Company A of the 105th Pennsylvania Infantry in Pittsburgh on September 9.

[5] Ordered to defensive duties near Washington, D.C. in early October, the Mitchell brothers and their fellow 105th Pennsylvanians were transported south by railroad, and pitched their tents at Camp Kalorama on the Kalorama Heights outside of Georgetown, District of Columbia, before relocating again to Alexandria, Virginia, where they were assigned to the brigade led by Brigadier-General Charles Davis Jameson in the U.S. Army division commanded by Brigadier-General Samuel P. Heintzelman.

Reassigned to duties at Fortress Monroe on March 17, 1862, they then engaged in their first significant combat during the Siege of Yorktown (April 5–May 4) and Peninsula Campaign (March–July 1862), including the battles of Williamsburg (May 5) and Fair Oaks/Seven Pines (May 31), during which James Mitchell's older brother, Alex, was wounded in action.

[6][7] In his battle report, Jameson described how the day's events unfolded:[8] All our men had fled from the abattis [sic] in the vicinity of the Richmond Road.

This is a state of perfection in discipline that is gained by but few regiments.Afterward, James Mitchell continued to participate in operations against Confederate troops in Virginia from the late summer through early winter.

Three days after Christmas, while encamped at Brandy Station, James Mitchell re-enlisted for service as a private with Company A,[12] and was awarded a veterans' furlough.

Departing winter quarters on May 4, 1864, the Mitchell brothers and their regiment were assigned to the Overland Campaign of Lieutenant-General Ulysses S. Grant, which immediately brought them under fire in the Battle of the Wilderness (May 5–7).

[14][15] While his brother recuperated, James Mitchell and their regiment continued to engage with the enemy, including during the battles of North Anna (May 23–26) and Cold Harbor (May 31–June 12).

[2][18] Serving there until 1900, he introduced legislation in 1897 to ensure that city bakeries and small factories producing baked goods would be subject to state factory inspections, that schools statewide would have uniform textbooks which were pre-approved by a gubernatorial commission, and which transferred responsibility for the maintenance and rebuilding of township bridges of more than thirty feet in length to the commissioners of the counties where those bridges were located.