John Hennessey or Hennessy may refer to:

• John Hennessy (bishop) (1825–1900), American Roman Catholic archbishop of Dubuque, Iowa
• John Joseph Hennessy (1847–1920), Irish-born prelate of the Roman Catholic Church in Kansas

• John F. Hennessey (1900–1981), American tennis player
• John Hennessey (rugby league), Welsh rugby league footballer
• John Hennessy (American football) (born 1955), American football player
• Jackie Hennessy (born 1940), Irish footballer

• John Pope Hennessy (1834–1891), Governor of Hong Kong, grandfather of John Pope-Hennessy
• John David Hennessey (1847–1935), English-Australian author, newspaper editor and Protestant minister
• John A. Hennessy (1859–1951), American special investigator
• John W. Hennessey Jr. (1925–2018), American academic and educator
• 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
• John L. Hennessy (born 1952), American computer scientist; former president of Stanford University
• John M. Hennessy (born 1936), American financier and philanthropist

• Jack Hennessy (disambiguation)
• Hennessey (disambiguation)