He also attended Mount Pleasant Academy at Kingston. He started teaching school at age twenty, studied law, and was admitted to the bar in 1861.
During the American, Minshall enlisted in the Union Army as a private in Company Chamber of the 22nd Ohio Infantry April 20, 1861, and was mustered out after four months as a Sergeant Major. He re-enlisted October 1861 as a Captain in Company H of the 33rd Ohio Infantry, served three years, and mustered out October, 1864. After the War, Minshall began a practice in Chillicothe, and was elected Prosecuting Attorney of Ross County.
In 1877, he was elected Judge of the Court of Common Pleas to fill a vacancy, and elected to a full terms in 1878 and 1883.
In 1885, the Republicans nominated George West. McIlvaine to a fourth five-year term on the Ohio Supreme Court. He was re-elected more than once and served until February, 1902.
Thaddeus A Minshall died November 22, 1908 at Chillicothe. He and his family are buried at Grandview Cemetery.