John Forsyth was born on April 2, 1827, in Pennsylvania. He eventually moved to Ohio, and he married Martha Dickson on November 10, 1849. They had at least four children: Sarah, born around 1850; Rosalia, born around 1855; Maggie, born around 1857; and John, born around 1866. They lived in Rich Hill, Ohio, and he earned a living as a farmer. In 1860, he was renting his home, and he owned $150 of personal property.
He enlisted in the Union army on August 9, 1862, and he mustered in as a private in Company G of the 97th Ohio Infantry on September 1. The regiment took part in the Battle of Perryville, the Battle of Stones River, the Tullahoma campaign, and the Battle of Missionary Ridge. In July 1864, army officials transferred him to the 1st United States Veteran Volunteer Engineers.
He returned to Rich Hill after the war and resumed his work as a farmer. By 1870, he owned $10,000 of real estate and $1,100 of personal property. He applied for a federal pension in January 1890 and eventually received one. His wife died in 1899. By 1900, he was living with his daughter Maggie and his granddaughter in Salt Creek, Ohio. He died of apoplexy in Rix Mills, Ohio, on April 2, 1907.