Hugh was born on the 6th of April 1856, in Fond du Lac, Wisconsin, where he grew up with one brother and one sister. He attended the US Military Academy and commissioned as a Second Lieutenant upon graduation in June of 1880. Assigned to the 4th US Cavalry, Hugh also attended the Infantry and Cavalry Schools at Fort Leavenworth, Kansas, and was an instructor of military science at Wisconsin University for three years. By June of 1897, he had been promoted to Captain and served in New Mexico and Arizona during campaigns that would fall under the American Indian Wars. During the Spanish-American War, Hugh was promoted to Major of Engineers in the Volunteer service and was assigned to Jacksonville and Savannah, before heading to Havana with the Army’s Seventh Corps. He then deployed to Manila in the Philippines in support of the Philippine Insurrection, and it was his actions one month after arriving that would earn him the Medal of Honor. The citation reads:

Swam the San Juan River in the face of the enemy's fire and drove him from his entrenchments.

When American troops were in front of the town of Calamba, it became necessary to cross the river to take the town. The river had risen from recent rain and the only boats that existed in the area were two canoes on the opposite side of the river. A witness of Hugh’s actions stated:

“He did not wait for orders, nor did he call for volunteers.  He stripped and plunged into the whirling stream and came back in half an hour with two canoes.  There were some bullet holes in the canoes by the time he got across with them, but they were made to serve the purpose of transporting a storming party across the stream and the trench was taken.”

Three months later, on the 8th of October 1899, Hugh was in a mission between the towns of Bacoor and Imus when the enemy fired a cannon loaded with scrap iron at the American movement. He took an iron nut to his left thigh and was taken to a hospital after the battle. He had sent a cable to his wife, Mary, that he had been wounded but not to worry. Mary was his second wife; his first was Lillian with whom they had one son. One month after being wounded, on the 7th of November 1899, Hugh Jocelyn McGrath died at the hospital at the age of 43. He posthumously received the Medal of Honor on the 29th of April 1902, and is buried in Arlington National Cemetery: Section 1, Lot 315-ES.