Isaac Underwood (Lewis) was born around 1799 in Albemarle County, Virginia. He worked as a farmer, although it is unclear if he was free or enslaved. He enlisted in the Union army on April 12, 1864, in Nashville, Tennessee, for a period of three years. He mustered in as a private in Company B of the 101st USCT Infantry Regiment on June 20 and served on detached duty as a teamster for contraband collected around camp. The army discharged him for disability on June 18, 1865, in Clarksville, Tennessee, and he died of smallpox at the general hospital in Nashville on Novemer 25, 1865.
Underwood was married at the time of his death, although the name of his wife remains unknown. His son, Edward Underwood, attempted to file a minor pension in 1888 for his father’s service. An auditor at the War Department recommended that Edward receive some compensation as Isaac’s son, but the pension agency ultimately declined his application. They cited inconsistencies in his application, observing that Edward had gotten his father's name and regiment wrong. Edward eventually refiled his claim with the correct information. Because Isaac had died after his discharge, however, the pension agency ruled that Edward did not qualify for a pension.