Moseley was born in Evanston, Illinois, on September 28, 1874. He graduated from the United States Military Academy in 1899 and was commissioned second lieutenant in the cavalry. He served in the Philippines twice, from 1900 to 1903 and 1906 to 1907, where his assignments included commanding a troop of the 1st Cavalry and serving as Aide-de-Camp to Generals J. M. Bell and J. M. Lee. In 1901 Moseley, accompanied by only one other officer, without escort and under conditions of great danger, penetrated a major Philippine insurgent stronghold. 2nd Lt. Moseley and 1st Lt. George Curry convinced Brigadier General Ludovico Arejola to sign the peace agreement in Taban, Minalabac (Philippines) on 25 March 1901.