Although Porter served well in the early battles of the Civil War, his military career was ruined by the controversial trial, which was called by his political rivals.

He served in the Mexican–American War and was appointed a brevet captain on September 8, 1847, for bravery at the Battle of Molino del Rey.

[4] He was an original member of the Aztec Club of 1847, a military society composed of officers who served during the Mexican War, and presided over it from 1892 to 1893.

Afterward, Porter inspected and reorganized the defenses of Charleston Harbor, South Carolina, until late 1860, when he aided the evacuation of military personnel from Texas after that state seceded from the Union.

[5] After the start of the Civil War, Porter became chief of staff and assistant adjutant general for the Department of Pennsylvania, but he was soon promoted to colonel of the 15th Infantry on May 14, 1861.

[6] In August, Porter was promoted to brigadier general of volunteers, backdated to May 17[4] so he would be senior enough to receive divisional command in the Army of the Potomac, newly formed under Maj. Gen. George B. McClellan.

In addition, Porter had a memorable experience when he decided to make aerial observations in a hydrogen balloon without the assigned expert to handle the craft, Professor Thaddeus Lowe.

Fortunately, the combination of a favorable wind change and himself adjusting the gas valves allowed Porter to return to the Union lines and land safely.

[8] Although it was an embarrassing accident, General Porter was able to perform his observations of enemy defences as intended and recorded his findings, although the balloon program was disbanded a year later.

As the V Corps turned to head towards Jackson's right and attacked, it presented its own (and consequently the entire army's) flank to Longstreet's waiting men.

Porter's association with the disgraced McClellan and his open criticism of Pope were significant reasons for his conviction at court-martial.

In 1878, a special commission under General John Schofield exonerated Porter by finding that his reluctance to attack Longstreet probably saved Pope's Army of Virginia from an even greater defeat.