M. L. Armstrong, Texas legislator, was born August 13, 1805, in the future Overton County, Tennessee, son of Col. James and Nancy Lanier Armstrong. Colonel Armstrong died when Micajah was young, leaving him to be raised "in moderate circumstances" by his mother. Without formal education Armstrong established a plantation in his home state of Tennessee and was elected to the Tennessee State Legislature. In 1852 M. L. Armstrong immigrated to Texas, where he settled in Lamar County and lived as a planter near Paris.
He was elected to the House of the Eighth Texas Legislature from Lamar County in 1859. During his term Armstrong spoke against disunion and during 1860 published a circular in opposition to a secession convention. Despite his Unionist tendencies Armstrong took an oath during the Civil War to support the Confederacy. Armstrong was the Lamar County delegate to the Constitutional Convention of 1868–69, where he was considered by some an advocate for the rights of freedmen. In 1869 M. L. Armstrong was elected to the House of the Twelfth Texas Legislature. He was sworn in on February 10, 1870, but on February 19 was declared ineligible to serve due to his oath against the United States government. After being replaced in the state legislature Armstrong served as both county clerk and district court clerk for Lamar County. Armstrong's political disabilities were removed in 1871.
According to some sources M. L. Armstrong was married four times. It is possible to confirm his marriages to Sarah Leanah McMillin, who died in 1855, and Eliza Kirkpatrick, who died in 1887. Micajah Armstrong lived in Lamar County until his death on June 13, 1893. He is buried in the Old City Cemetery in Paris.