John Hennessy (August 20, 1825 – March 4, 1900) was a 19th-century Irish-born prelate of the Roman Catholic Church who served as bishop and archbishop in the United States.

He was consecrated and installed bishop in St. Raphael's Cathedral on September 30, 1866, by Archbishop Peter Richard Kenrick of Saint Louis.

[2] In 1878, at his suggestion, the Sisters of the Third Order of St. Francis of the Holy Family relocated from Iowa City to Dubuque to staff a German Catholic Orphan Asylum.

Archbishop Kenrick and the other bishops of the province advocated splitting Iowa into two with a diocese in the east (Dubuque) and one in the west (Council Bluffs).

[2] During his tenure as bishop and archbishop, Hennessy oversaw the expansion of the diocese even though this was a period of anti-Catholic sentiments in Dubuque and across the United States.

The Ecclesial Province of Dubuque included the dioceses of Davenport, Omaha, Wichita and Sioux Falls.

The remains of Bishops Mathias Loras, Clement Smyth, and Hennessy were brought to the cathedral and buried in this new chapel.

A stained glass window in St. Joseph Church in Sioux City, Iowa, contains a portrait of Archbishop Hennessy.