He was brought up in large houses requisitioned by the local council, first in Allandale Avenue and then in Lyndhurst Gardens, Finchley, north London.

[3] He attended the nearby Our Lady of Lourdes Primary School, and on Sundays he went to St Mary Magdalene Church, where he was an altar boy.

[3] He was the subject of the first episode, first broadcast on 6 August 2007, of the BBC Radio 4 series The House I Grew Up In, in which he talked about his childhood.

In June 1977, Hennessy accused Donald Beves of being the "fourth man" in the Cambridge Spy Ring (then-known participants were Philby, Burgess, and Maclean), but Geoffrey Grigson and others quickly leapt to the defense of Beves, considering him uninterested in politics.

On the subject of the Metropolitan Police fines issued to Boris Johnson for lockdown breaches during the Partygate scandal, he said "I think we're in the most severe constitutional crisis involving a prime minister that I can remember.

From 1992 to 2000, he was professor of contemporary history at Queen Mary and Westfield College, University of London.

[7] On 5 October 2010 the House of Lords Appointments Commission said that Hennessy was to be a crossbench (non-political) peer.

"[10] In August 2014, Lord Hennessy was one of 200 public figures who were signatories to a letter to The Guardian opposing Scottish independence in the run-up to September's referendum.