Curley was born in Tremane, near Athleague in County Roscommon, Ireland, on May 8, 1825, and received a common school education. He immigrated to the United States in 1851, and settled at first in St. Louis, Missouri, where he became an active member and officer of several militia companies. He entered the military service in 1860, as a first lieutenant in the Missouri Volunteer Militia's Southwest Battalion, and served for six months on the frontier of the state. In June 1861, after the outbreak of the Civil War and the Camp Jackson Affair, he enlisted in the United States Army, and was commissioned a major in the 7th Missouri Volunteer Infantry, known as the "Irish Seventh". Curley was promoted in May 1862 to lieutenant colonel, and in July of that year he was sent home to Missouri to recruit, raising in a short time the 27th Missouri Volunteer Infantry, of which he was made colonel. He participated in the Vicksburg Campaign, the capture of Jackson, the campaign of the XV Army Corps from Vicksburg to Chattanooga, and the battles of Lookout Mountain and Missionary Ridge. He was with William Tecumseh Sherman in his march to the sea; fought at Resaca, Dallas, Kennesaw Mountain, Jonesborough, and Lovejoy's Station, and in the capture of Savannah and Fort McAllister. He was in the campaign through South Carolina which captured Charleston and Columbia; was in on the battle of Bentonville, capture of Raleigh, and many minor campaigns.