Fletcher was born in Herculaneum, Missouri. His parents had immigrated to Missouri from Maryland in 1818. He received a public school education and was elected circuit clerk in Jefferson County, Missouri, from 1849 until 1856. He was admitted to the bar in 1857. Fletcher became a land agent for the southwest branch of the Pacific Railroad (which later became the St. Louis and San Francisco Railway) whereupon he moved to St. Louis. Although he had been raised as a Democrat in a slave-owning family, he had been an ardent abolitionist since his boyhood and became a Republican after 1856.