Gilmor was born at "Glen Ellen", the Jacobethan/English Tudor-styled "Castle" family estate, near the village of Warren,, just north of Towsontown in central Baltimore County, Maryland. He was the son of Robert Gilmor and Ellen Gilmor, daughter of Judge William H. Ward. Harry was the fifth of eleven children.
As part of the third major Confederate invasion of the North, this under commanding Gen. Jubal Early with several corps of troops on a mission to attack the national capital at Washington, D.C. and possibly liberate Southern prisoners-of-war at Camp Point Look-Out in southern Maryland at the confluence of the Chesapeake Bay and the Potomac River in St. Mary's County. After the Battle of the Monocacy, along the Monocacy River on July 9, 1864, southeast of Frederick in Frederick County, Maryland, Colonel Gilmor's command, along with Frederick's Brig. Gen. Bradley T. Johnson's Maryland Confederate infantry and cavalry, made a series of raids around Baltimore going as far east as Magnolia Station in Harford County, Maryland and Fork, Maryland. On July 10, 1864, Major Harry Gilmor of the 2nd Maryland Cavalry was given 135 men of the 1st and 2nd Maryland, and directed to cross northern Baltimore County into Harford County at Jerusalem Mill, and destroy the railroad bridge of the Philadelphia, Wilmington, and Baltimore Railroad at Magnolia Station, across the Gunpowder River, northeast of the city, in Harford County. In the meantime, while crossing his home county, he stopped and visited his family at the family estate "Glen Ellen", then near the former village of Warren, now beneath the surface of Loch Raven Reservoir. Early on July 11, Gilmor's advance group passed the home of Ishmael Day on Sunshine Avenue in Fork. Day, a strong Union sympathizer, had hung a large United States flag to greet Gilmor's troops. Sergeant Eugene Fields, a member of the advance guard unit, told Day to take the flag down. Day refused, so Sgt. Fields dismounted to do it himself. Day shot Field at close range with a shotgun. Day immediately fled, hiding under an apple cider press for days until the passing troops were gone. Gilmor's men then burned Day's home and barn. Maj. Gilmor sent Sgt. Field to Wright's Hotel, where he later died. At about 8:40 in the morning on July 11, Gilmor's cavalrymen reached the station and proceeded to stop two northbound trains from Baltimore. After evacuating the passengers, the troopers set fire to the second train and backed it down the tracks and onto the bridge. The train burned through the draw section of the bridge and effected much damage to the area around it. Aboard the first train was a convalescing Union Maj. Gen. William B. Franklin. This raid was regarded as one of the most daring during the war by detached cavalry on either side.
After the war, Gilmor moved to New Orleans, where he married Miss Mentoria Nixon Strong, daughter of Jasper Strong and Eliza Julia Nixon. Gilmor and his wife had three children. Gilmor wrote his war memoirs, entitled "Four Years in the Saddle". He soon returned to Maryland and was elected a colonel of the cavalry in the reorganized Maryland National Guard. He also served as the Baltimore City Police Commissioner from 1874 to 1879,. Gilmor died in Baltimore, plagued by complications from a war injury to his jaw. He was buried in Loudon Park Cemetery in the southwest section of the city between Frederick Road and Wilkens Avenue, in an area of the cemetery now known as "Confederate Hill." At his death, Baltimore City Police Department central and district stations flew their flags at half-staff. Gilmor's funeral was a large local ceremonial event with many dignitaries present to honor this war hero. There is also a Gilmor Street laid out between Calhoun Street running south to Cole Street in West Baltimore's Upton, Sandtown-Winchester, Poppleton neighborhoods.
