Samuel Peter Heintzelman, United States Army officer and mining engineer, was born in Manheim, Pennsylvania, on September 30, 1805, the son of Peter and Ann Elizabeth (Grubb) Heintzelman. After receiving a fair primary education he was appointed to the United States Military Academy at West Point on July 1, 1822. He graduated seventeenth in his class and was commissioned a second lieutenant in the Second Infantry on July 1, 1826. His earliest assignments were on the Canadian border and against the Indians in Florida. He was promoted to first lieutenant on March 4, 1833, and to captain on July 7, 1838. From that date until June 18, 1846, he served as regimental quartermaster. Heintzelman was married on December 5, 1844, to Margaret Stewart of Albany, New York. He served with Winfield Scott's army in Mexico and was brevetted to the rank of major on October 9, 1847, for "gallant and meritorious service" in the battle of Huamantla. After the Mexican War he was stationed in California, where he fought Indians and was promoted to major of the First Infantry on March 3, 1855. Heintzelman founded Fort Yuma, Arizona, at the junction of the Gila and Colorado rivers and commanded the post for four years, during which he saw considerable action against the Indians along the Rio Grande border. In 1858 and 1859 he served as president of the Sonora Exploring and Mining Company and director of its field operations south of Tucson. He was transferred to Texas in 1859, saw duty at Fort Duncan and Camp Verde, and participated in the campaign against Juan N. Cortina in 1859–60.
With the outbreak of the Civil War Heintzelman was promoted to colonel of the Seventeenth Infantry, on May 14, 1861. He was appointed a brigadier general of volunteers three days later and commanded a division at the federal debacle at first Manassas (first Bull Run), where he was severely wounded. On May 5, 1862, he was promoted to major general and given command of the Third Corps of Gen. George B. McClellan's army. He was brevetted to brigadier general in the regular army on May 31, 1862, for "gallant and meritorious conduct" at the battle of Fair Oaks, Virginia, and reassigned to the army of Gen. John Pope, then operating in northern Virginia. Completely outgeneraled by Thomas J. (Stonewall) Jackson, Heintzelman and Pope met inglorious defeat at the second battle of Manassas, whereupon Heintzelman was assigned to the defenses of Washington, D.C. In 1864 he commanded the Northern Division, the central United States, for a brief time but spent the remainder of the war on court-martial duty. He was promoted to major general of volunteers on March 13, 1865, for "gallant and meritorious service" at the battle of Williamsburg, Virginia. General Heintzelman retired from the volunteer service on August 24, 1865, but maintained command of the Seventeenth Infantry and served with it, mostly in Texas, until his retirement on February 22, 1869. Subsequent to his retirement he was promoted to major general. He died in Washington, D.C., on May 1, 1880, and is buried in Buffalo, New York.