Harden Green Keplinger was born on November 25, 1839, in Franklin, Illinois, to Samuel Keplinger and Permelia Green. His father was a farmer who owned $16,500 of real estate and $1,800 of personal property in 1860. Keplinger grew up and attended school in Franklin before enrolling at Illinois College.
He left school in April 1861 to enlist in the Union army, and he mustered in as a private in Company B of the 10th Illinois Infantry. He mustered out three months later at the end of his term of enlistment. He enlisted again on August 27, 1861, mustering in as a private in Company A of the 32rd Illinois Infantry later that day. The regiment took part in the Battle of Shiloh. He was promoted to 1st lieutenant on October 14, 1862, and transferred to the 122nd Illinois Infantry. He participated in the Battle of Paducah and the Battle of Nashville, and he mustered out on July 15, 1865.
Keplinger returned to Illinois after the war, and he married Matilda Bell on October 3, 1867. They had at least two children: Maurice, born around 1871; and Ada, born around 1873. His wife died around 1875. In 1880, Keplinger was working as a laborer in Deadwood in the Dakota Territory. He returned to Franklin in the late 1800s, and by 1900, he was working as a banker. A decade later, he had become a bank president. He applied for a federal pension in April 1904 and eventually secured one. He died in Franklin on June 9, 1921.