Whilst a student, Moorhouse worked as a researcher for Professor Norman Davies, collaborating on many of the latter's best-known publications, including Europe: A History, The Isles: A History and Rising '44 and culminating in the publication in 2002 of a co-authored study of the history of the city of Wrocław (the former German Breslau) entitled Microcosm: Portrait of a Central European City.
In 2006 Moorhouse's first solo book, Killing Hitler, was published, which has since been translated a number of times. In a CNN news report of 3 September 2011, Killing Hitler was shown on Al-Saadi Gaddafi's desk after he had fled his office in the wake of the collapse of the Gaddafi regime in Libya. Reporter Nic Robertson suggested that Saadi Gaddafi had been reading the book prior to his flight.
His next book, Berlin at War, is a social history of Berlin during World War II, which was published in the summer of 2010. Writing in the Financial Times, Andrew Roberts commented that: "Few books on the war genuinely increase the sum of our collective knowledge of this exhaustively covered period, but this one does". "Berlin at War" was listed amongst the books of the year for 2010 by The Daily Telegraph, and American Spectator magazine, among others, and was shortlisted for the Hessell-Tiltman Prize for history.
In 2014 his The Devils' Alliance: Hitler's Pact with Stalin, 1939-1941 was published. Whilst praising the book for its "masterly" account of the signing of the Molotov–Ribbentrop Pact, historian Richard J. Evans took exception to the book's "unbalanced treatment" of the crimes of the Soviets over those of the Nazis, asserting that "for all its virtues this is a deeply problematic book". Other reviewers of the book were more positive – the Wall Street Journal described it as "superb" and The Daily Telegraph listed it among its Books of the Year for 2014.
A fluent German speaker, Moorhouse is a specialist in modern German history, particularly on Adolf Hitler and the Third Reich. In this capacity, he has written for The Times, The Independent on Sunday, and the Financial Times, and is a regular contributor to both the BBC History magazine and History Today. Increasingly, Moorhouse is now concerning himself with modern Polish history, especially of the wartime period.
Moorhouse is a regular public speaker, and has appeared, among others, at the Edinburgh International Book Festival and the Bath Literature Festival. Since 2016, he has been a visiting professor at the College of Europe in Natolin near Warsaw. He also conducts educational tours of Germany and Poland, organised by Historical Trips Ltd. He is a Fellow of the Royal Historical Society.
- Jak powstało Powstanie '44, Norman Davies, (Znak, Kraków, 2005), Chapter
- The Men Who Tried to Kill Hitler, Roger Manvell & Heinrich Fraenkel, (Frontline Books, 2008), Introduction
- He Was My Chief: The Memoirs of Adolf Hitler's Secretary, Christa Schroeder, (Frontline Books, 2009), Introduction
- With Hitler to the End: The Memoir of Hitler's Valet, Heinz Linge, (Frontline Books, 2009), Introduction
- The Celebrated Pedestrian and Other Historical Curiosities, (BBC History, 2009), Contributions
- I was Hitler's Chauffeur: The Memoirs of Erich Kempka, Erich Kempka, (Frontline Books, 2010), Introduction
- The Hitler I Knew, Otto Dietrich, (Skyhorse Publishing, 2010), Introduction
- The Hand of History: An Anthology of Quotes and Commentaries, Michael Leventhal (ed.), (Frontline Books, 2011), Contribution
- While Berlin Burns, Hans-Georg von Studnitz, (Pen & Sword Books, 2012), Introduction
- The Sniper Anthology, Martin Pegler (Ed.), (Frontline Books, 2012), Chapter on Simo Häyhä
- Hitler was My Friend: The Memoirs of Hitler's Photographer, Heinrich Hoffmann, (Frontline Books, 2012), Introduction (ISBN 978-1-84832-608-8)
- The Devils' Alliance: Hitler's Pact with Stalin, 1939-1941, (Bodley Head, 2014) ISBN 978-1847922052
- The Third Reich in 100 Objects, (Greenhill Books, 2017)
- Roger Moorhouse’s website – “born 1968 Stockport, Cheshire
- "Video News - CNN". CNN.
- Financial Times, 7 August 2010, Andrew Roberts, 'Battle for Berlin'
- "PEN Hessell-Tiltman Prize - English PEN".
- Evans, Richard J. (6 August 2014). "The Devils' Alliance: Hitler's Pact with Stalin, 1939-1941 – review". The Guardian. Retrieved 30 October 2014.
- Simms, Brendan (30 November 2014). "Comrades in Arms" – via online.wsj.com.
- Rahim, Sameer (15 April 2015). "The best books of 2014" – via www.telegraph.co.uk.
- mtc. "The History that Shaped Us - Historical Trips". www.historicaltrips.com.