Leonard Willard Carpenter was born on January 26, 1834 in Indiana, Pennsylvania.
On April 27, 1861 Carpenter mustered in as First Lieutenant in Company A of the 4th Ohio Infantry Regiment when it was initially organized for ninety days service. When the regiment reorganized for three years on June 4, 1861 he became captain of Company A. He was promoted to major on December 1, 1862 and lieutenant colonel on December 13. He commanded the regiment at Gettysburg on July 1-3, 1863.
He was promoted to colonel on November 1 but his muster was revoked as the regiment had fallen below the minimum number of men required for a full colonel. Carpenter continued to command the regiment until it mustered out on June 21, 1864.
After the war Carpenter lived in Mount Vernon and Columbus, Ohio and New Albany and Logasport, Indiana. He studied to become a physician at the Cleveland Homeopathic Medical College, graduating in 1876. In 1888 he moved to Seattle, Washington.
Carpenter died on February 18, 1908 in Seattle. He is buried in Gettysburg National Cemetery.