David Hennessy joined the New Orleans police force as a messenger in 1870. While only a teenager, he caught two adult thieves in the act, beat them with his bare hands, and dragged them to the police station. He made detective at the age of 20. With his cousin Michael Hennessy, he arrested the notorious Italian bandit and fugitive Giuseppe Esposito in 1881. Esposito was wanted in Italy for kidnapping a British tourist and cutting off his ear, among numerous other crimes. Esposito was deported to Italy, where he was given a life sentence. In 1882, Hennessy was tried for the murder of New Orleans Chief of Detectives Thomas Devereaux. At the time, both men were candidates for the position of chief. Hennessy argued self-defense and was found not guilty. Hennessy left the department afterwards and joined a private security firm given police powers by the city. He handled security for the New Orleans World Fair of 1884–1885. The New York Times noted that Hennessy’s men were, “neatly uniformed and are a fine-looking and intelligent body of men, far superior to the regular city force.” In 1888, Joseph A. Shakspeare, the nominee of the Young Men’s Democratic Association, was elected mayor of New Orleans with Republican support. Having promised to end police inefficiency, Shakspeare promptly appointed Hennessy as his police chief. David Hennessy inherited a police force that was (according to the local press) incompetent and plagued by corruption. Under his supervision, it began to show signs of improvement.

On the evening of October 15, 1890, David Hennessy was shot by several gunmen as he walked home from work. Hennessy returned fire and chased his attackers before collapsing. When asked who had shot him, Hennessy reportedly whispered to Captain William O’Connor, “Dagoes.” Hennessy was awake in the hospital for several hours after the shooting, and spoke to friends, but did not name the shooters. The next day complications set in and he died. There had been an ongoing feud between the Provenzano and Mantranga families, who were business rivals on the New Orleans waterfront. Hennessy had put several of the Provenzanos in prison, and their appeal trial was coming up. According to some reports, Hennessy had been planning to offer new evidence at the trial which would clear the Provenzanos and implicate the Mantrangas. If true, this would mean that the Mantrangas, and not the Provenzanos, had a motive for the murder. A policeman who was a friend of Hennessy’s later testified that Hennessy had told him he had no such plans. In any case, it was widely believed that Hennessy’s killers were Italian. Local papers such as the Times-Democrat and the Daily Picayune freely blamed “Dagoes” for the murder.