Eastman was born in Brooklyn circa 1873, relocated to Lower Manhattan circa 1895, and became one of the "Sheriffs" of New Irving Hall. The corrupt Tammany Hall political machine regularly relied on the 1200+ member Eastman Gang to "Get Out the Vote" at election time. He engaged in a protracted turf war versus Paul Kelly of the Five Points Gang gang. Later, at age 44, he joined the 106th Infantry of the 27th Division of the U.S. Army, "O'Ryan's Roughnecks", and fought in France during World War I. He was killed by a Prohibition agent shortly after his return.
He is featured in a 1933 fictional short story by Jorge Luis Borges called "Monk Eastman: Purveyor of Iniquities". In the P.G. Wodehouse novel, "Psmith In The City", the fictional character Bat Jarvis is largely based on him, sharing his kindhearted streak with regard to animals