The administration building to the east is three stories high, with a brick-and-limestone facade, a large arch facing Lexington Avenue, and a double-height mansard roof; there are various offices and other rooms inside.

As of 2023[update], it is still used as the headquarters of the New York Army National Guard's 1st Battalion, 69th Infantry Regiment, as well as for special events.

[19] The center of the Lexington Avenue elevation contains a massive limestone round arch, surrounded by brick headers and topped by an eagle-shaped keystone.

[16][17] Local firm Milliken Brothers manufactured the steelwork for the roof, which is supported by six arches,[22][23] each consisting of two three-hinged steel trusses.

[25] After the American Civil War ended, the New York state government passed a law, which mandated that armories be erected for volunteer regiments by each of the individual counties.

[34][37] Another site at the intersection of Lexington Avenue and 23rd Street, occupied by a City College of New York building,[a] was identified by 1896[38] but was also rejected.

[34][37] Colonel Edward Duffy and other regimental officers said the City College site, at 200 by 225 ft (61 by 69 m), would be too small for a drill hall.

[43] In October 1900, the city appointed Horgan & Slattery to design the 69th Regiment Armory;[44][45] the relatively unknown firm had connections to the politically powerful Tammany Hall organization of the time.

[46] That December, Brigadier-General McCoskry Butt expressed his opposition to the design,[47][48] but the Armory Board nonetheless approved the plans the next month.

[49] Horgan and Slattery filed plans for the armory in October 1901, which were to cost $450,000 and contain a drill hall, gymnasium, shooting range, pool, seven-story lookout tower, and other rooms for the regiment.

[50][51] After Seth Low won the 1901 New York City mayoral election, he declared his intention to break up Horgan & Slattery's monopoly on municipal buildings.

[53][54] That March, Low moved to cancel the board's contract with Horgan & Slattery,[54][55] and he appointed a committee to help the regiment find suitable temporary quarters.

[74] Mayor George B. McClellan Jr. laid the armory's cornerstone at a groundbreaking ceremony on April 23, 1904,[75][76] marking the 43rd anniversary of when the 69th Regiment left New York City to fight in the Civil War.

[22][79] That November, the city's Sinking Fund Commission approved $18,500 in bonds for lockers, gun racks, and railings at the armory.

[80] The armory hosted its first event, a vehicular show, in January 1906,[81] but its official opening date was repeatedly rescheduled.

[88][89] By then, local civic groups wanted the 69th Regiment to open up the armory to the community during the summers due to a lack of nearby public parks.

[94] The armory's drill hall hosted a wide variety of events, becoming what The New York Times called "Manhattan's multipurpose site".

[106] As part of a 1936 Works Progress Administration project, Earl Lonsbury painted some murals in a meeting hall in the basement.

[107] Another plaque at the armory was dedicated in early 1940, commemorating members of the 165th Infantry's machine-gun company who had been killed in World War I.

[108] With the onset of World War II, the New York state government stopped renting out the armory for civilian events that year.

[114] Despite being a frequent venue for basketball games, in 1967, the 69th Regiment Armory was described by one critic as "close to being a fire trap" and unpopular among regular visitors.

[128] The first event at the armory, the Automobile Club of America's vehicular show, opened on January 13, 1906,[129][130] and attracted tens of thousands of people in a week.

[128][132] Although two-thirds of the featured artists were American,[89] it also attracted Europeans like Vincent Van Gogh, Pablo Picasso, Henri Matisse, Raoul Dufy, Marcel Duchamp, and Andre Dunoyer de Segonzac.

[89] Due to its novel nature, the show attracted large amounts of commentary when it opened,[132][134] and it was still being discussed decades after it closed.

[137][138] The armory held its first-ever "open games" in April 1907, with members of several regiments participating in a variety of races.

[148] Thure Johansson of Sweden broke Dorando Pietri's indoor record for the marathon at the 69th Regiment Armory on March 1, 1910 (2 hours, 36 minutes, 55.2 seconds).

[178] During its first few decades, the armory hosted pageants,[179] speeches,[180] bazaars,[181] and local civic meetings,[182] as well as its first "municipal dance" in 1914.

[189] Also during the 1920s, two newspapers owned by publisher William Randolph Hearst gave away free food to needy citizens every year during Christmas.

[191] During the 1960s, the armory hosted such varied events as an "African Carnival" fundraiser,[192] electronic music performances,[193] a political rally,[194] and an engineering-themed art festival.

[203] Additionally, the armory hosted the Downtown Art Fair starting in 2014,[204] and it has sometimes been used for the Victoria's Secret Fashion Show.