Born in East Greenwich, RI on July 12th, 1807, Casey was accepted into the U.S. Military Academy just 11 days before his 15th birthday.   Casey graduated from West Point on July 1, 1826, ranked 40th in a class of 42.  with a brevet to 2nd Lieutenant, 7th Infantry and a commission of the same date to 2nd Lieutenant, 2nd Infantry. He served on Frontier Duty at Ft. Towson, RI, 1826-1829, being engaged in a Skirmish with hostile Indians, September 2, 1828; in garrison at Sackett's Harbor, NY, 1829- 1832; on Recruiting Service, 1832-1833; in garrison at Ft. Niagara, NY, 1833-1834 and Ft. Gratoit, MI, 1834-1836. He was promoted to 1st Lieutenant, 2nd Infantry, June 28, 1836.

Casey continued on Frontier Duty in the Creek Nation, 1836; in the Florida War against the Seminole Indians, 1837-1842, being engaged leading the advance in the Rout of Halleck Tustennuggee's band in the Big Hammock of Pilaklikaha, April 19, 1842; in garrison at Buffalo, NY, 1842-1845, and Ft. Mackinac, MI, 1845-1847. He was promoted to Captain, 2nd Infantry, July 1, 1839.

He was in the War with Mexico, engaged in the Skirmish of Ocalaca, August 16, 1847; Battle of Contreras, August 19-20, 1847; and Battle of Churubusco, August 20, 1847. He received a brevet to Major, August 20, 1847, for gallant and meritorious conduct in the Battles of Contreras and Churubusco.

Casey was next engaged in the Battle of Molino del Rey, September 8, 1847, and the Storming of Chapultepec, September 13, 1847, where he was severely wounded while leading the assault column under his command. He was breveted to Lieutenant Colonel, September 13, 1847, for gallant and meritorious conduct in the Battle of Chapultepec. The legislature of Rhode Island voted him its thanks for meritorious service during the Mexican War.

He next served in garrison at Ft. Hamilton, NY, 1848 and on a voyage via Cape Horn to California, 1848-1849. Casey was on Frontier Duty at Benicia, CA, 1849; escorting Capt. Warner's Topographical party, 1849; at Benicia, CA, 1849-1851; on the Expedition to Port Orford and Coquille River, OR, 1851-1852; again at Benicia, CA, 1852; on Recruiting Service, 1852-1854; and served as a member of a Board of Officers, 1854-1855, to revise the "Rifle and Light Infantry Tactics", adopted March 29, 1855 for the service of the United States. He was promoted to Lieutenant-Colonel, 9th Infantry, March 3, 1855.

Casey served as a member of the Board for Dragoon Manual for Colt's Revolver, 1854; and of the Board for the examination of Breech-loading Arms, 1854-1855; on Frontier Duty, in command of Puget Sound District, WA, January 17, 1856-February 19, 1857, being engaged in Operations against the hostile Indians, March-June, 1856; at Ft. Steilacoom, WA, 1856-1859; San Juan Island, WA, 1859; and Ft. Steilacoom, WA, 1859-1861.

He was commissioned a Brigadier-General, U. S. Volunteers, August 31, 1861 and was engaged in organizing, disciplining and instructing volunteers at Washington, D. C., September 12, 1861-March 28, 1862. He was promoted to Colonel, 4th Infantry, October 9, 1861.

In the Virginia Peninsular Campaign, he was engaged in the Battle of Fair Oaks (Seven Pines), May 31, 1862, his division of Erasmus D. Keyes' IV Corps bearing the brunt of the first Confederate attack by A. P. Hill's troops. Casey's Redoubt at Seven Pines was named for him. Even though his position was overrun, he was breveted to Brigadier- General, U. S. Army, May 31, 1862, for gallant and meritorious conduct, and was promoted to Major-General, U. S. Volunteers. He received the thanks of the Rhode Island Legislature for his "bravery, skill, and energy" at the Battle.

Casey was next placed in command at White House, VA, June 1862; in command of Provisional Brigade, at Washington, D. C., August 11, 1862-February, 1863; as President of the Board for the Examining Candidates for Officers of Colored Troops, May 22, 1863-July 5, 1865. He was breveted to Major-General, U. S. Army, March 13, 1865, for gallant and meritorious services during the Rebellion.

Following the Civil War, he served in command of a regiment and troops at Ft. Wayne and Detroit, MI, October 2, 1865-April 5, 1867; as a Commissioner to examine the War Claims of Ohio, April 17- December 31, 1867; on a Court of Inquiry, 1868; and as a Member of the Retiring Board, New York City, October 28, 1868-April 26, 1869. He retired from active service July 8, 1868.

Casey compiled and edited a system of "Infantry Tactics" (based upon the French "Ordonnances" of 1831 and 1845), which was adopted for the service of the United States, August 11, 1862; and of "Infantry Tactics for Colored Troops", adopted March 9, 1863. He died at Brooklyn, NY on January 22, 1882 and was buried on the Casey family farm at North Kingstown, RI.