A native of Maine, Morrill was educated in public school and after briefly attending Waterville College served as principal of a private school in New York. He studied law and passed the bar in 1839, afterwards setting up law practices in Readfield and Augusta, Maine. Morrill, known for his eloquent speaking, soon become popular among Democratic friends advocating temperance. Morrill was elected to Maine's House of Representatives in 1854 as a Democrat and served as Chairman of the Maine Democratic Party. However, as the nation divided over slavery during the 1850s, Morrill's politics changed and he went over to the Republican Party for the sole reason that the Republicans were opposed to the expansion of slavery. He was elected Maine's state senator in 1856 as a Republican, and elected Governor of Maine in 1858, serving until 1861 during the outbreak of the American Civil War. Morrill was elected Maine's U.S. Senator in 1861 when a vacancy opened in the U.S. Senate, after Sen. Hannibal Hamlin assumed the office of Vice President under President Abraham Lincoln. Morrill's extended tenure for almost 15 years as U.S. Senator took place during the Civil War and Reconstruction. Morrill sponsored legislation that outlawed slavery in Washington D.C. and advocated education and suffrage for African American freedmen.