David C. Hennessy (1858–October 16, 1890) was an American policeman and detective who served as a police chief of New Orleans from 1888 until his death in 1890. As a young detective, he made headlines in 1881 when he captured a notorious Italian criminal, Giuseppe Esposito. In 1888, he was promoted to superintendent and chief of police. While in office he made a number of improvements to the force, and was well known and respected in the New Orleans community.

His assassination in 1890 led to a sensational trial. A series of acquittals and mistrials in March 1891 angered locals, and an enormous mob forced open the prison doors and lynched 11 of the 19 Italian men who had been indicted for Hennessy's murder in the largest known mass lynching in U.S. history.

David C. Hennessy was born in New Orleans in 1858, to Margaret and David Hennessy Sr., Irish Catholics living on Girod Street. David Sr. was a member of the First Louisiana Cavalry of the Union Army during the U.S. Civil War, formed after the state was occupied by Union troops. After the war, during the Reconstruction era, he served with the Metropolitan Police, a New Orleans force under the authority of the governor of Louisiana. Local white Democrats generally considered the Metropolitan Police as a military occupation army, in part because it protected the right of freedmen to vote, in accordance with the Fifteenth Amendment. David Sr. was murdered in 1869 by Arthur Guerin, a fellow policeman.

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 and private detectives James Mooney and John Boland of New York City, 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 for 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.

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.