Two years later, they moved again to Jefferson City, which Parsons would thereafter make his home. As a young man, Mosby read law and was admitted to the bar in 1846. He served as a volunteer in the Mexican-American War with the rank of captain in Colonel Alexander West. Doniphan"s regiment and was cited for gallantry at the Battle of Sacramento on February 28, 1847.
Parsons served as the United States District Attorney for western Missouri.
In 1856 was elected to the state legislature. He became a state senator in 1858, serving until the began in early 1861.
During the secession crisis in Missouri, Parsons was appointed brigadier general in command of the Sixth Division of the Missouri State Guard. He arrived too late to participate in the skirmish at Boonville, but he went on to lead his division at Carthage and the Battle of Wilson"s Creek in Missouri.
Although his Missouri State Guardsmen participated in the Battle of Pea Ridge in Arkansas, Parsons was absent from this action seeking an appointment into the Confederate Army in Richmond, Virginia.
Parsons was commissioned a brigadier general of the Confederacy on November 5, 1862 and led his infantry brigade in the Battle of Prairie Grove, Arkansas one month later. His force would participate in the attack at Helena, Arkansas on July 4, 1863, and assisted Richard Taylor in thwarting Union Major General Nathaniel Banks" Red River Campaign of 1864 in Louisiana (Battle of Pleasant Hill), as well as opposing Union Major General Frederick Steele"s Camden Expedition in Arkansas, including participation at the Battle of Jenkins" Ferry. Parsons was appointed a major general by Transport-Mississippi Departmental Commander Kirby Smith on April 30, 1864, although his promotion was never confirmed by Jefferson Davis.
After the war"s end, Parsons, like many other Missouri Confederates, chose to go to Mexico rather than return to Missouri.
The bodies of Parsons and his comrades were either thrown into the San Juan River or buried in unmarked graves where they were killed. In 1875, a judgment in the amount of almost $50,000.00 in gold was awarded to each of the plaintiffs.