Brummer started his media career at J Walter Thomson and Haymarket Publishing between 1970 and 1972. He then went to work for The Guardian as the Financial Correspondent. He was the main reporter on the fringe banking crisis of 1973/4 and the 1976 sterling crisis.

In 1979 he became the US Financial and Washington Correspondent for the Guardian. He covered the 1980, 1984, and 1988 U.S. presidential elections for The Guardian. His work in this area earned him the 1989 Overseas Press Club award for the best foreign correspondent in the US.

Brummer then took up positions as a Foreign Editor and Financial Editor, and completed his twenty-six year tenure at the Guardian as Associate Editor.

He worked as Consultant Editor for the Financial Mail on Sunday between 1999 and 2000 and was voted Financial Journalist of the Year at the British Press Awards. In 2000 he became the City Editor of the Daily Mail. Brummer covered the 2003 Iraq War for the Daily Mail from Washington, D.C.

Brummer led the Daily Mail's coverage on the 2007 run on Northern Rock, collapse of Lehman Brothers, and subsequent credit crunch.

On 4 February 2009, Brummer appeared as a witness at the House of Commons Treasury Select Committee, along with Robert Peston (BBC), Lionel Barber (Financial Times), Simon Jenkins (Guardian), and Sky News Business Editor Jeff Randall to answer questions on the role of the media in financial stability and "whether financial journalists should operate under any form of reporting restrictions during banking crises". He is a member of the editorial advisory board of Jewish Renaissance magazine.

Brummer has a degree in economics and politics from the University of Southampton and an MBA from the University of Bradford Management Centre.