Longstreet was born in Augusta, Georgia, a son of the inventor William Longstreet. He graduated at Yale University in 1813, studied law in Litchfield, Connecticut, and was admitted to the bar in Richmond County, Georgia. He soon moved and rose to eminence as a lawyer in Greensboro, Georgia. He represented Greene County in the state legislature in 1821, and in 1822 became a district judge in Ocmulgee. After several years as a judge, he declined re-election and resumed his legal practice in Augusta, did editorial work, and established the Sentinel, which soon merged with the Chronicle (1838). In 1838, he became a Methodist minister. During this period of his ministry, the town was visited with yellow fever, but he remained at his post, ministering to the sick and dying.
In 1839, he was made president of Emory College. After nine years he accepted the presidency of Centenary College, Louisiana, then of the University of Mississippi, where he stayed for six years, after which he resigned, and became a planter, but in 1857 became president of South Carolina College. Just before the Civil War, he returned to his old presidency in Mississippi.
In politics he belonged to the Jeffersonian school of strict construction and states rights. He made speeches on all occasions through his life. “I have heard him,” writes one who knew him, “respond to a serenade, preach a funeral sermon, deliver a college commencement address, and make a harangue over the pyrotechnic glorifications of seceding states. He could never be scared up without a speech.”
During his years as a Southern Methodist minister Longstreet preached a doctrine of secession and defended slavery. He was conspicuous in the discussions that led to a rupture of his church. Scholar Lewis M. Purifoy notes that "Augustus B. Longstreet, in a baccalaureate address to the University of South Carolina graduating class of 1859, urged the young men of his audience to defend Southern rights to the utmost. While they should not strive to break up the Union, they should not ‘make a dishonorable surrender of the thousandth part of the mill more to save it.’ He defended slavery mainly on the ground that freeing [slaves] would be ruinous to Southern society; and the burden of his speech was that the South had suffered long and grievously at the hand of the North. Longstreet assured the class that secession would not lead to war, but, if it should, a united South would win.
At an early age, he began to write for the press, and his pen was never idle. His chief periodical contributions are to be found in The Methodist Quarterly, Southern Literary Messenger, The Southern Field and Fireside, The Magnolia, and The Orion, and include “Letters to Clergymen of the Northern Methodist Church” and “Letters from Georgia to Massachusetts.” His fame is based, however, on a single book, of which he was the author: Georgia Scenes (1835), originally published in newspapers, then gathered into a volume at the South, and finally issued in 1840 in New York. It featured realistic sketches of Southern humor. It is said that he disavowed the second edition (1867) and tried to destroy the first.
Augustus was a mentor for his nephew James Longstreet, and was a long time friend and associate of John C. Calhoun. He died in Oxford, Mississippi and is buried in section one of St. Peter's Cemetery.
American actress Lara Parker is his third-great-granddaughter.