Brendan Simms is the son of Anngret and David Simms, a professor of mathematics at Trinity College Dublin. He studied at TCD, where he was elected a Scholar in 1986, before completing his doctoral dissertation, Anglo-Prussian relations, 1804–1806: The Napoleonic Threat, at Cambridge under the supervision of Tim Blanning in 1993. A Fellow of Peterhouse, he lectures and leads seminars on international history since 1945.
In addition to his academic work, he also serves as the president of the Henry Jackson Society, which advocates the view that supporting and promoting liberal democracy and liberal interventionism should be an integral part of Western foreign policy.
Brendan Simms is a grand-nephew of the leading Church of Ireland ecumenist and scholar, former Archbishop of Dublin and Armagh, George Otto Simms. He is also a grand-nephew of Brian Goold-Verschoyle, a member of the Communist Party of Ireland, who became a Soviet spy and died in a Soviet gulag in 1942. Simms himself was raised in the Catholic faith. 
Simms's research focuses on the history of European foreign policy. His overarching book, Europe: The Struggle for Supremacy, 1453 to the Present, was favourably reviewed by The Telegraph and the New Statesman
- The Struggle for Mastery in Germany, 1779–1850 (Palgrave MacMillan, 1998)
- Unfinest Hour: Britain and the Destruction of Bosnia (Penguin, 2001)
- Three Victories and a Defeat: The Rise and Fall of the First British Empire, 1714–1783 (Penguin, 2007)
- Europe: The Struggle for Supremacy, 1453 to the Present (Allen Lane, 2013)
- The Longest Afternoon, The Four Hundred Men who Decided the Battle of Waterloo (Allen Lane, 2014)
- Britain's Europe: A Thousand Years of Conflict and Cooperation (Penguin, 2017)
- Hitler: A Global Biography (Basic Books, 2019) ISBN 978-0465022373
- Historian says Varadkar has breached letter and spirit of the Belfast Agreement, irishtimes.com, 19 July 2019
- "List of Scholars". TCD Life. Retrieved 20 December 2015.
- Brendan Simms' webpage at the Department of Politics and International Studies, Cambridge University
- "Professional Staff". Henry Jackson Society. Retrieved 2014-11-16.
- "Statement of Principles". Henry Jackson Society. Retrieved 2014-11-16.
- Brendan Simms , theguardian.com
- Europe by Brendan Simms: review, Noel Malcolm, The Telegraph, 15 April 2013
- Reviewed: Europe – the Struggle for Supremacy by Brendan Simms, Norman Stone, New Statesman 25 April 2013