Isaac Stevens began his military career after graduating first in his class from the United States Military Academy in 1839. He was commissioned as an Officer in the Corps of Engineers, and spent several years constructing forts in New England. He served as adjutant for the Corps of Engineers during the Mexican-American War, and served during the battles of Cerro Gordo, Contreras, Churubusco, and during the Siege of Vera Cruz.
He received a brevet promotion to Captain, and led troops during the Battle of Chapultepec, where he received a brevet promotion to Major. He fought during the Battle of Molino del Rey, and was severely wounded at the Battle of Mexico City. After the war, he was named Governor of the Washington Territory in 1853, and was elected the territorial delegate for Congress in 1856.
At the outbreak of the Civil War, Stevens was commissioned as a Colonel of the 79th New York, known as the Highlanders. He was promoted to Brigadier General on September 28, 1861, and led troops during the Port Royal Expedition. He led a division during the Battle of Secessionville, but after suffering heavy casualties, was transferred to Virginia. He took command of the IX Corps, and led them during the Second Battle of Manassas. On September 1, 1862, Isaac Stevens was killed instantly while leading his men at the Battle of Chantilly.
He was buried in Newport, Rhode Island at Island Cemetery. In March 1863, he was posthumously promoted to major general, backdated to July 18, 1862. Fort Stevens, located in Washington DC and part of the Civil War Defenses of Washington, was named in his honor.
Hazard Stevens, Isaac's son, was also injured in the Battle of Chantilly. He also became a general in the U.S. Army and an author, and along with P. B. Van Trump participated in the first documented ascent of Mount Rainier.