Philip Johnston was born on April 27, 1837, in Knox County, Ohio. By the early 1860s, he was working as a farmer in Lexington, Illinois.
He enlisted in the Union army on August 12, 1862, and he mustered in as a private in Company C of the 94th Illinois Infantry eight days later. According to his service records, he was 5 feet, 8 inches tall, with light hair and blue eyes. The regiment took part in the siege of Vicksburg, and he eventually earned a promotion to corporal. He mustered out on July 17, 1865.
He returned to Illinois after the war, and he married Elizabeth Sipe around 1866. They had at least two children: May, born around 1871; and Benjain, born around 1872. He worked as a farmer in Blue Mound, Illinois, and he employed at least one farm laborer. He applied for a federal pension in April 1890 and eventually received one. In March 1905, he began suffering from paralysis. Later that year, on November 16, 1905, he was admitted to the National Home for Disabled Volunteer Soldiers. He was discharged two days later at his own request. A local writer described him as “one of the most highly respected men in this vicinity, a man of strong character and sterling worth [who] enjoyed the confidence and respect of all who knew him.” He died of “kidney trouble” on January 15, 1912.