Kiernan’s father is a retired British navy surgeon. Kiernan attends Trinity College, Dublin, before emigrating to the United States around 1854. He studies medicine at New York University and practices law in New York until 1861. Upon the outbreak of American Civil War in 1861 he joins the 69th New York State Infantry Regiment as Assistant Surgeon and serves as such through the First Battle of Bull Run. When the 69th returns to Manhattan, he moves west and becomes the surgeon of the 6th Missouri Volunteer Infantry.
After the Battle of Pea Ridge, Kiernan insists on joining the fighting ranks, and in that capacity is seemingly appointed a Major in the 6th Missouri Volunteer Infantry. In May 1863 at Port Gibson, Mississippi, he is wounded in the left lung and left on the battlefield for dead. Recovered and imprisoned, he effects an escape back to Union forces and resigns his commission. On August 1, 1863 he is commissioned a Brigadier General of the United States Volunteers by President Abraham Lincoln, commanding a post at Miliken’s Bend on the Mississippi River. However, ill-health as a result of his battlefield wounds force him to resign on February 3, 1864.
In May 1865 Kiernan gains a U.S. consular post at Chinkiang, China. He manages to make the trip there but his health does not allow him to perform the duties. He returns to New York where he becomes an examining physician for the Bureau of Pensions, a position he holds until his death.
James Lawlor Kiernan dies on November 26, 1869. The official cause of death is “congestion of the lungs.” Perhaps he is killed by that Confederate ball that wounded him six years earlier. He is buried in Green-Wood Cemetery in Brooklyn, New York.