Horrocks joined the BBC in October 1981 as a news trainee. He went on to work at Newsnight as an assistant producer and then producer. After time as a senior producer, intake editor and output editor on Breakfast Time, he became deputy editor of Panorama in 1988.
Horrocks edited BBC television's General Election results coverage in 1992. He edited the coverage of the Budget, by-elections and local elections, as well as the 1994 European Election results and General Election results programmes in May 1997.
In May 1992, Horrocks was appointed editor of BBC Two's social affairs programme, Public Eye, a position he held until he launched Here And Now, a current affairs magazine intended to capture high audience figures, in January 1994. He became editor of Newsnight in April 1994, and editor of Panorama in December 1997. Horrocks became Head of Current Affairs in June 2000. He was executive producer of Brits, True Spies, Smallpox 2002, The Day Britain Stopped, Dirty War and of the documentary trilogy The Power of Nightmares.
Horrocks won Bafta awards in 1997 and 2005 for his editorship of Newsnight and for the The Power of Nightmares.
He became Head of Television News in September 2005. In November 2007, following a restructuring of BBC News, he became Head of the BBC newsroom. 
In April 2009, he replaced Nigel Chapman at BBC World Service and has been responsible for the overall editorial leadership and management of the world's leading international multimedia broadcaster since then.