The son of Isaac Mendenhall and Elizabeth Atkisson, John was born near Mt. Vernon, Illinois and moved to Iowa around 1849. He enlisted as a private on August 23, 1861 in Farmington, Iowa, and joined Company B, 3rd Iowa Cavalry seven days later in Keokuk. Regimental records noted that he was 5'7" with blue eyes and brown hair. He served with the regiment in Missouri and Arkansas and was present for the Battle of Pea Ridge, but took ill soon after, reporting to a field hospital in Cassville, Missouri on April 4. He remained there until July 11, when he transferred to the New House of Refuge hospital in St. Louis. He was discharged from the hospital on January 20, 1863, with the examining surgeon noting that John had "hypertrophy of the heart very well marked with tremendous shock of the same." He married America Ellen Fridley on June 26, 1863 in Clark County, Missouri and they had at least seven children: Willie, Laura Alice, Mary Elizabeth, Nellie, Lillian Maude, Carrie and Georgia Frances. They moved from Iowa to California sometime between 1880 and 1900.

His obituary in the April 13, 1908 issue of the Sacramento Bee:

"LIVE OAK (Sutter County)- April 13 - John W. Mendenhall, who was stricken with paralysis two weeks ago, died Saturday. He was a native of Mt. Vernon, Ill and was nearly 69 years of age. He was a veteran of the Civil War, a mamber of the G.A.R., A.O.U.W. and Masonic fraternities. He leaves a widow and seven of the twelve children born to survive him; Mrs. A. Perry, Live Oak, Mrs. W.W. Drullinger, Fair Oaks; Mrs. J. A. Whipple, Santa Rosa, Mrs. W.H. McPeak, Gridley and Miss Georgia Mendenhall of Sacramento. The remains were taken to Sacramento yesterday, where the funderal will take place at The Masonic Temple."