John Warrington Caldwell was born around 1840 in Pennsylvania to John and Sarah Warrington. His father was a farmer who owned $4,000 of real estate and $1,000 of personal property by 1860. The family lived in Philadelphia, Pennsylvania, until the 1850s, when they moved to Chester, Pennsylvania. By 1860, he was working as a clerk.
He enlisted in the Union army on August 7, 1862, and he mustered in as a private in Company C of the 15th Pennsylvania Cavalry on August 22. The regiment took part in the Battle of Antietam and the Battle of Chickamauga. He became a quartermaster sergeant on March 1, 1863. He voted for Abraham Lincoln in the election of 1864, noting, "my cry is still for war." He mustered out on June 21, 1865.
Caldwell settled in Philadelphia after the war. He married Anna Flickwir in the early 1870s, and they had at least four children: John, born around 1871; Mary, born around 1874; Lilly, born around 1877; and Warrington, born around 1880. Caldwell worked as an umbrella salesman. He died in Philadelphia on November 25, 1900.