Horatio Collins King was born on December 22, 1837 in Portland, Maine to Horatio and Anne Collins King. The elder King served as postmaster general in the cabinet of James Buchanan. In 1854 the younger King entered Dickinson College, where his uncle Charles Collins was president. He earned his bachelor's degree in 1858. Following graduation, King studied law for two years with Edwin M. Stanton (later Secretary of War) and in 1861 moved to New York City. He was admitted to the New York State Bar that same year.

When the Civil War erupted, King sought a commission in the United States Army. In 1862 he received from his former mentor Stanton an appointment as assistant quartermaster of volunteers with the rank of captain in the Army of the Potomac. He was soon promoted to chief quartermaster of the First Cavalry Division of the Army of the Shenandoah. He took part in five battles following this appointment, and he was recommended for promotion because of gallantry at the Battle of Five Forks. King was honorably discharged in October 1866 with the brevets of major, lieutenant colonel, and colonel. He returned to the practice of law in New York City until 1871, when he assumed the position of associate editor at the New York Star. King then became publisher of the Christian Union with his close friend, Reverend Henry Ward Beecher, as editor. He also helped to edit The Christian at Work. In 1874 King returned to his law practice and remained active in the profession for the remainder of his life.

King joined the National Guard of New York in 1876 and was elected major of the Thirteenth Regiment. He was appointed judge advocate for the Eleventh Brigade in 1880 and, in 1883, was appointed by Governor Grover Cleveland to be judge advocate general, with the rank of brigadier general, in the National Guard, State of New York. A lifelong advocate of Civil War veterans' groups, he served as secretary of the Society of the Army of the Potomac from 1877 to 1904 and as president of that organization in 1904. He was a Mason, a member of the Order of Elks, and a charter member of the New York Commandery of the Military Order of the Loyal Legion. King was also an active member of the Grand Army of the Republic, serving two years as post commander and one year as department judge advocate general.

A Democrat, he served for ten years as a member of the Brooklyn Board of Education and a member of the New York Monuments' Commission. King ran unsuccessfully for Secretary of State of New York in 1895 on the Democratic ticket. He then ran for Congress in 1896 for the Sound Money Party, but was again defeated. When again nominated for office, King declined. In 1897, Horatio Collins King was awarded the Congressional Medal of Honor for "conspicuous gallantry" while serving with the cavalry in March 1865 near Dinwiddie Court House.

King married Emma Carter Stebbins, daughter of New York merchant Russell Stebbins, in October 1862. The union was not a long one as Emma died in childbirth in 1864. In June 1866 King married Esther Augusta Howard (1845-1925), the daughter of Captain John T. Howard with whom he had served during the War. He and Esther had nine children and resided in Brooklyn, New York for much of their lives. Horatio Collins King died on November 15, 1918 in Brooklyn.

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