He studied law, was admitted to the bar in 1848 and commenced practice in Meadville, Pennsylvania. He was a delegate to the 1860 Republican National Convention. Public service  On March 21, 1861, President Lincoln appointed Pettis an associate justice of Colorado Territory, but he never really served in the position. He departed Denver some time after July 30, 1861 without ever presiding over the court he was appointed to. He remained absent until his replacement, Allen A. Bradford was appointed associate justice in June 1862. He returned to Meadville and continued to practice law. Pettis was elected as a Republican to the Fortieth Congress to fill the vacancy caused by the death of Darwin A. Finney. He was an unsuccessful candidate for reelection in 1868. He resumed the practice of law in Meadville. Pettis was appointed Minister to Bolivia September 4, 1878, and served until November 1, 1879.
He studied law, was admitted to the bar in 1848 and commenced practice in Meadville, Pennsylvania. He was a delegate to the 1860 Republican National Convention. Public service
 On March 21, 1861, President Lincoln appointed Pettis an associate justice of Colorado Territory, but he never really served in the position. He departed Denver some time after July 30, 1861 without ever presiding over the court he was appointed to. He remained absent until his replacement, Allen A. Bradford was appointed associate justice in June 1862. He returned to Meadville and continued to practice law. Pettis was elected as a Republican to the Fortieth Congress to fill the vacancy caused by the death of Darwin A. Finney. He was an unsuccessful candidate for reelection in 1868. He resumed the practice of law in Meadville. Pettis was appointed Minister to Bolivia September 4, 1878, and served until November 1, 1879.