Mosby Monroe Parsons was born on May 21, 1822, in Charlottesville, Virginia, United States. He was the son of Gustavus Adolphus Parsons and Patience Monroe Bishop. At age 13 his family moved to Cole County, Missouri, and two years later to Jefferson City, Missouri.
Parsons attended St. Charles College, Missouri, and also studied law.
Mosby Parsons began his practice in Jefferson City in 1846. During the Mexican War, he participated in the invasion of California and was promoted to captain. Active in Missouri Democratic politics, he was elected attorney general of Missouri in 1853 and served until 1857. He represented Cole County in the Missouri House in 1857 and was elected to the state Senate in 1859.
When the Civil War began, Parsons organized the Missouri State Militia. He fought under General Sterling Price at the battles of Carthage, Wilson’s Creek, and Lexington, Missouri, and was a hero of the battle of Elkhorn, Arkansas, in March 1862.
After his promotion to brigadier general on November 5, 1862, he served in the Arkansas campaign of 1862-1863 and in Taylor's Red River campaign in Louisiana in 1864. His able performance at the battle of Pleasant Hill in April 1864 won Parsons's recommendation for promotion to major general. Although he was never confirmed, he was assigned to the duties of this rank from April 30, 1864, until the end of the war.
After the war, Parsons along with other Missouri Confederates chose to go to Mexico rather than surrender. On August 15, 1865, Parsons and five companions were captured and executed by bandits near Camargo, Chihuahua.
Mosby Parsons is known as a brigadier general who was placed in command of the Sixth Division of the Missouri State Guard. Parsons led his division in the battles of Carthage and Wilson's Creek in Missouri and Pea Ridge in Arkansas.
During the American Civil War, he led an infantry brigade at the Battle of Prairie Grove, Arkansas, he participated in the attack on Helena, Arkansas, and assisted in thwarting Union General Nathaniel Banks'' Red River Campaign of 1864 in Louisiana.
A secessionist, Parsons allied with Governor Claiborne Jackson in attempting to persuade his state to secede.
Mosby married Mary Wells on September 18, 1850.
