He attended the West Chester Normal School, then entered the United States Military Academy and graduated number sixty-one of sixty-two in the class of 1892,[1] one of thirteen who took five years to complete the requirements.

Mearns participated in the Battle of El Caney and earned the Silver Star.

[2][5] Mearns was promoted to major and commanded the Philippine Scouts from February 28, 1905, to December 8, 1909.

During World War I, he received a temporary promotion to colonel on August 16, 1917, and to brigadier general on October 12, 1918.

[4] From November 1, 1918, to February 5, 1919, he served as commanding general of the 17th Infantry Division at Camp Beauregard in Louisiana.