He is primarily known for his travel writing and for his biographies, where he has covered subjects as diverse as Diego Rivera, Georges Simenon, Jean Moulin and Mary Wesley.
Marnham was educated at St Philip's, a Catholic day school in Kensington, then by the Benedictines of Downside and at Corpus Christi College, Oxford where he read Jurisprudence.
He was called to the Bar by the Benchers of Gray's Inn in 1966 but instead of embarking on a legal career become a reporter for Private Eye, where he shared an office with both Paul Foot and Auberon Waugh.
As a script writer for the BBC TV satire programme At The Eleventh Hour, his colleagues included Roger McGough, Miriam Margolyes, Richard Neville, Leonard Rossiter, Esther Rantzen and Stephen Frears, among others.
The crime caused a bitter quarrel between Lord Lucan's friends that led to the suicide of the impecunious artist and flaneur Dominic Elwes.
Instead, a serious drought had been skilfully transformed by a consortium of development experts in USAID, FAO and various British NGOs to undermine the West African economy and increase their own influence in the region.
At that time, he led a campaign with Richard West and Auberon Waugh for the installation of a British monument to honour those repatriated to Soviet concentration camps as a result of the Yalta Conference.
[1][2] He left The Spectator to travel in Mexico and through the war zones of Guatemala, El Salvador and Nicaragua, an experience which he described in So Far from God: A Journey to Central America.
Marnham's travel writing was described in the Dictionary of Literary Biography as covering "complex cultural histories" and tackling "substantial questions about belief, skepticism, communal responsibility and individual freedom...
In the tradition of such anatomizers of late British imperialism as Graham Greene, Malcolm Lowry and Evelyn Waugh, Marnham documents with tragic irony and self-deprecating wit the fate of parts of the world that were once administered - and are still in many ways controlled - by Europe and the United States...
It marked a return to travel writing and described journeys to the Democratic Republic of the Congo, New Mexico and Japan, tracing the story of the atomic bombing of Hiroshima and Nagasaki.