Jonathan Sproul was born around 1839 in Monmouth County, New Jersey, to James and Julia Sproul. His father was a carpenter and farmer who owned $7,000 of real estate and $1,200 of personal property by 1860. Sproul grew up and attended school in Raritan, New Jersey, and by 1860, he was working as a farmer. He moved to Keyport, New Jersey, around 1860 and began work as a clerk.
He enlisted in the Union army in Brooklyn, New York, on September 20, 1861, and he mustered in as a sergeant in Company C of the 87th New York Infantry on October 2. According to his service records, he was 5 feet, 10 inches tall, with light hair and blue eyes. He took part in the siege of Yorktown, the Battle of Williamsburg, the Battle of Seven Pines, the Seven Days Battles, and the Second Battle of Bull Run. The army transferred him to Company C of the 40th New York Infantry on September 6, 1862, and his new regiment participated in the Battle of Antietam, the Battle of Fredericksburg, the Battle of Chancellorsville, and the Battle of Gettysburg.
He remained devoted to the Union, writing in September 1862 that “our prospects are still bright and the cause of Liberty shall yet prevail. And the fabric our fathers reared is founded on to[o] firm a foundation to be uterly [sic] overthrown after nearly a century’s growth.” He was wounded in action on November 7, 1863, and surgeons amputated his left leg. He never fully recovered, and he died in Washington, D.C., on December 7, 1863.