Robert N. Earhart was born in Blairsville, Pennsylvania on April 9, 1833 to merchant David Earhart and his wife, Catharine Altman Earhart. His father took his large family, of which Robert was the youngest, to Pleasant Valley, Iowa during the 1840s. The younger Earhart received his preparatory education at Alexander College, a Presbyterian institution in Dubuque, Iowa that closed in 1857. He then returned to his native state for his undergraduate degree, enrolling at Dickinson College in Carlisle in the autumn of 1854. Earhart was elected to the Belles Lettres Society and graduated with his class in 1858.
Earhart then attended the B.D. Garrett Biblical Institute in Evanston, Illinois and qualified in 1860 as a clergyman of the Methodist Episcopal Church. He returned to his home area in Iowa and joined the Upper Iowa Conference of that church, where he served congregations for the remainder of his active life. His pastorates included churches at Toledo in Tuma County, at Osage in Mitchell County, and at the First Methodist of Manchester in Delaware County. After forty-one years of service to northern Iowa, he retired from the pulpit in 1901.
Earhart married Frances Fidlar, a native of Columbus, Ohio who had grown up in Davenport, Iowa. They had one child, Robert, who later graduated from Northwestern and became a college professor. Robert Nixon Earhart died in Davenport, Iowa on August 29, 1907 and was buried in the Oakdale Cemetery there. He was seventy years old.