Longstreet's talents as a general made significant contributions to the Confederate victories at Second Bull Run, Fredericksburg, and Chickamauga, in both offensive and defensive roles. He also performed strongly during the Seven Days Battles, the Battle of Antietam, and until he was seriously wounded, at the Battle of the Wilderness. His performance in semiautonomous command during the Knoxville Campaign resulted in a Confederate defeat. His most controversial service was at the Battle of Gettysburg, where he disagreed with General Lee on the tactics to be employed and reluctantly supervised the disastrous infantry assault known as Pickett's Charge.

He enjoyed a successful post-war career working for the U.S. Government as a diplomat, civil servant, and administrator. However, his conversion to the Republican Party and his cooperation with his old friend, President Ulysses S. Grant, as well as critical comments he wrote in his memoirs about General Lee's wartime performance, made him anathema to many of his former Confederate colleagues. Authors of the Lost Cause movement focused on Longstreet's actions at Gettysburg as a primary reason for the Confederacy's loss of the war. His reputation in the South was damaged for over a century and has only recently begun a slow reassessment.

Longstreet was born in Edgefield District, South Carolina (in an area that is now part of North Augusta in Edgefield County). He was the fifth child and third son of James and Mary Ann Dent Longstreet, originally from New Jersey and Maryland respectively, who owned a cotton plantation close to where the village of Gainesville would be founded in northeastern Georgia. James's ancestor Dirck Stoffels Langestraet immigrated to the Dutch colony of New Netherland in 1657, but the name became Anglicized over the generations. James's father was impressed by his son's "rocklike" character on the rural plantation, giving him the nickname Peter, and he was known as Pete or Old Pete for the rest of his life.

James's father decided a military career for his son, but felt that the local education available to him would not be adequate preparation. At the age of nine, James was sent to live with his aunt and uncle in Augusta, Georgia. His uncle, Augustus Baldwin Longstreet, was a newspaper editor, educator, and a Methodist minister. James spent eight years on his uncle's plantation, Westover, just outside the city, while he attended the Richmond County Academy. His father died from a cholera epidemic while visiting Augusta in 1833; although James's mother and the rest of the family moved to Somerville, Alabama, following his father's death James remained with uncle Augustus.

In 1837 Augustus attempted to obtain an appointment for James to the United States Military Academy, but the vacancy for his congressional district had already been filled so James was appointed in 1838 by a relative, Reuben Chapman, who represented the First District of Alabama (where Mary Longstreet lived). James was a poor student academically and a disciplinary problem at West Point, ranking 54th out of 56 cadets when he graduated in 1842. He was popular with his classmates, however, and befriended a number of men who would become prominent during the Civil War, including George Henry Thomas, William S. Rosecrans, John Pope, D.H. Hill, Lafayette McLaws, George Pickett, John Bell "Sam" Hood, and his closest friend, Ulysses S. Grant of the class of 1843. Longstreet was commissioned a brevet second lieutenant in the 4th U.S. Infantry.

Longstreet spent his first two years of service at Jefferson Barracks, Missouri, where he was soon joined by his friend, Lieutenant Grant. Longstreet introduced Grant to his fourth cousin, Julia Dent, and the couple eventually married. Longstreet would serve as Grant's "best man" at the wedding. Soon after that introduction Longstreet met Maria Louisa Garland, called Louise by her family. She was the daughter of Longstreet's regimental commander, Lt. Col. John Garland. They were married in March 1848, after the Mexican-American War. Although their marriage would last for over 40 years and produce 10 children, Longstreet never mentioned Louise in his memoirs and most anecdotes about their relationship came to historians through the writings of his second wife.

Longstreet served with distinction in the Mexican War with the 8th U.S. Infantry. He received brevet promotions to captain for Contreras and Churubusco and to major for Molino del Rey. In the Battle of Chapultepec on September 12, 1847, he was wounded in the thigh while charging up the hill with his regimental colors; falling, he handed the flag to his friend, Lt. George E. Pickett, who was able to reach the summit.

After the war and his recovery from the Chapultepec wound, Longstreet and his new wife served on frontier duty in Texas, primarily at Fort Martin Scott near Fredericksburg and Fort Bliss in El Paso. He performed scouting missions and also served as major and paymaster for the 8th Infantry from July 1858. Author Kevin Phillips claims that during this period Longstreet was involved in a plot to draw the Mexican state of Chihuahua into the Union as a slave state.

Longstreet was not enthusiastic about secession from the Union, but he had learned from his uncle Augustus about the doctrine of states' rights early in his life and had seen his uncle's passion for it. Although he was born in South Carolina and reared in Georgia, he offered his services to the state of Alabama, which had appointed him to West Point and where his mother still lived. Furthermore, he was the senior West Point graduate from that state, which implied a commensurate rank in the state's forces would be available. He resigned from the U.S. Army in June 1861 to cast his lot with the Confederacy in the Civil War.

Longstreet arrived in Richmond, Virginia with a commission as a lieutenant colonel in the Confederate States Army. He met with Confederate President Jefferson Davis at the executive mansion on June 22, 1861, where he was informed that he had been appointed a brigadier general with date of rank on June 17, a commission he accepted on June 25. He was ordered to report to Brig. Gen. P.G.T. Beauregard at Manassas, where he was given command of a brigade of three Virginia regiments—the 1st, 11th, and 17th Virginia Infantry regiments.