John Theodore Heard was born around 1836 in Boston, Massachusetts, to John and Almira Heard. He graduated from Harvard University in 1859, and he earned a living as a physician in Boston.
In July 1861, he received a commission as an assistant surgeon in the 13th Massachusetts Infantry. He contracted typhoid fever in the fall of 1861, but he eventually recovered and rejoined the regiment. He was promoted to brigade surgeon in May 1862, and he became Medical Director of the Army of the Potomac’s First Corps in November 1862. Then, in April 1864, he became Medical Director of the Army of the Cumberland’s Fourth Corps. He mustered out on October 23, 1865.
He returned to Boston after the war and resumed his work as a physician. He married Rosalie Gaw, and they had at least three children: John, born around 1869; Henry, born around 1875; and Edmund, born around 1877. He employed at least three white domestic servants. He died of “organic (probably fibroid) heart disease” in Boston on September 2, 1906.