He attended Bristol County Academy, was graduated from Brown University in 1838, and from Harvard Law School in 1840.
After one year in the Boston office of Judge Peleg Sprague, he was admitted to the Suffolk bar in 1841 and practised in Boston for seventeen years. In 1858 he served in the state House of Representatives, where he was chairman of the committee on elections and rendered reports on important questions regarding election law, which the House came to follow. His judicial service began with his appointment in 1858 to the superior court of Suffolk County and continued unbroken for over thirty-two years.
Morton was by temperament an excellent judge, thorough, strong and reliable rather than brilliant, rapid in assimilating materials and in dispatching business, always accessible, of sufficient learning, courageous in deciding according to his convictions, and of unusual practical sagacity and native shrewdness.
Possessed of a direct and vigorous sense of justice, he viewed cases comprehensively, aiming at substantial justice rather than "the sharp quillets of the law". His summaries to juries were characterized by their simplicity, intelligibility, accurate sense of proportion, and impartiality.
His judgments, of which over twelve hundred are recorded in the Massachusetts Reports, are compact, clear, and forcible, and, in the opinion of his associates, contain few dicta which will require overruling or qualifications. As a nisi prius judge he is said to have had few equals in the history of the Commonwealth of Massachusetts.
In private life he was plain and unassuming and, though of great personal charm and popularity, averse to public display.
His first appearance in a public position was as a member of the Massachusetts Constitutional Convention of 1853, in which he sat for Andover, his home from 1850. During these years he was one of the original ten members of the state superior court, organized in 1859. Justice of the Supreme Judicial Court of Massachusetts from April 15, 1869.
And Chief Justice from January 16, 1882 to August 27, 1890, at which time he resigned because of ill health.
Married Abby B. Hoppin, October 19, 1843, 6 children.