Considered to be one of the few art collectors who was also a respected scholar, he is generally credited, alongside Sacheverell Sitwell and Tancred Borenius,[3] with bringing Italian pre-Baroque and Baroque painters to the attention of English-speaking audiences, reversing the critical aversion to their work that had prevailed from the time of John Ruskin.

[4] A lover of opera, he decided not to enter the family business but study art, spending a year working at the Ashmolean Museum under the supervision of Kenneth Clark.

[1] Serving as an honorary attache (informal, unpaid curator), he offered to buy Guercino's Elijah Fed By Ravens from the Barberini collection in Rome, and then sell it to the Gallery.

In the 1960s, Mahon and Sir Anthony Blunt became embroiled in a public feud over the iconography of the paintings of Nicolas Poussin, a subject in which both had published extensively and were recognised experts.

This came to a head in 1970 when Mahon attempted to pay the capital taxes on his deceased mother's estate by offering The Coronation of the Virgin by Annibale Carracci from his collection.

He was made a Member of the Order of the Companions of Honour (CH) in 2002 for his services to art, and received honorary doctorates from the universities of Newcastle, Oxford, Rome and Bologna.

[10] In a complex legal arrangement via his personal charitable trust, a number of these paintings (such as The Rape of Europa) are on permanent loan from The Art Fund, which owns them.

[12] After his death, the trustees of the charitable trust under instruction from Mahon's Will offered to donate the pieces to the national collection for free via The Art Fund, subject to the same conditions under which they were originally loaned.