DeLancey Floyd-Jones was born in South Oyster Bay, New York. He graduated from the United States Military Academy at West Point, 45th in the Class of 1846. With the outbreak of the Mexican–American War, he was assigned to Company D, 7th U.S. Infantry as second lieutenant. In November 1846, he was transferred to Company E, 4th U.S. Infantry. He participated in several major battles, including the Siege of Veracruz, the Battle of Molino del Rey, and the Battle for Mexico City. For gallant and meritorious conduct at Molino del Rey, he was brevetted to first lieutenant on September 8, 1847, receiving a promotion to the full rank in January 1848, when he was briefly assigned to duty in Pascagoula, Mississippi.