[2] By the time the American Civil War broke out, he was a civil engineer at Reading, Pennsylvania but chose to enlist at the 25th Pennsylvania Infantry Regiment but transferred to the 17th Infantry Regiment as a 1st Lieutenant.

He was then made part of Oliver Otis Howard's General Staff of the XI Corps and participated at the Battle of Wauhatchie, the Chattanooga Campaign and various skirmishes at the Atlanta Campaign with his horse being shot at the Battle of Jonesborough but was brevetted to lieutenant colonel on September 1, 1864.

He was transferred again to William Tecumseh Sherman's March to the Sea as part of the right wing and participated at the Battle of Bentonville.

[1] After the war, Pearson continued to serve at the 17th Infantry Regiment as a captain, initially stationing at Raleigh on the spring of 1870.

[2] Pearson also married his second wife, Maud Eskridge on April 16, 1898, at Fort Reno.