Beauregard excelled at West Point, graduating second in his class of 1842. His aptitude for engineering and tactics was evident from the start. His classmates included future Civil War adversaries.

Upon graduation, Beauregard was commissioned as a second lieutenant in the U.S. Army Corps of Engineers, a prestigious assignment reflecting his academic performance.

Beauregard served with distinction in the Mexican-American War under General Winfield Scott. He participated in several major battles, including the Siege of Veracruz and the Battle of Chapultepec, where he was wounded twice and brevetted for gallantry.

After the Mexican-American War, Beauregard was involved in various engineering projects, including the construction of fortifications along the Gulf Coast. This work further honed his skills and knowledge of defensive strategies.

Beauregard served as superintending engineer in New Orleans, overseeing the construction and maintenance of fortifications. This experience would prove valuable when he later commanded Confederate forces in the city.

Beauregard was briefly appointed Superintendent of West Point, but his tenure was short-lived due to the escalating secession crisis. This was a mark of high regard but quickly overtaken by events.

As Southern states began to secede, Beauregard resigned his commission in the U.S. Army to join the Confederate cause. His decision was based on his loyalty to his home state of Louisiana and his belief in states' rights.