John Hennessey or John Hennessy may refer to:
- John Hennessy (bishop) (1825–1900), American Roman Catholic clergyman; first archbishop of Dubuque, Iowa
- John Pope Hennessy (1834–1891), Governor of Hong Kong, grandfather of John Pope-Hennessy
- John David Hennessey (1847–1935), English author, newspaper editor and Protestant minister; in Australia since 1875
- John A. Hennessy (1859–1951), American special investigator from 1913
- John F. Hennessey (1900–1981), American tennis player; celebrated champion between 1923 and 1929
- John Hennessey (rugby league), rugby league footballer of the 1920s for Wales, and Rochdale Hornets
- John Pope-Hennessy (1913–1994), British art historian and museum director, grandson of John Pope Hennessy
- John J. Hennessey (1921–2001), American four-star army general; in charge of Readiness Command from 1974 to 1979
- Jackie Hennessy, Irish footballer
- John L. Hennessy (born 1953), American computer scientist; president of Stanford University since 2000
- John Hennessy (American football) (born 1955), American football player for the Michigan Wolverines and New York Jets
- John Joseph Hennessy (1847–1920), Irish-born prelate of the Roman Catholic Church in Kansas