Brought up in Hertfordshire and educated at Hitchin Boys' School, Bonfield graduated from Loughborough University with an engineering degree. He was Chief Executive of International Computers Ltd. from 1985 until the end of 1995. In 1996 Bonfield was appointed CEO and Chairman of the Executive Committee of British Telecommunications plc, where he served until early 2002. Promising a "rollercoaster ride", during Bonfield's tenure the share price went from £4 to £15, and back again to £5. Bonfield's salary to 31 March, 2001, was a basic of £780,000 (increasing to £820,000) plus a £481,000 bonus and £50,000 of other benefits including pension. He also received a deferred bonus, payable in shares in three years' time, of £481,000, and additional bonuses of £3.3 million. Andreas Whittam Smith writing in The Independent newspaper called Bonfield, Chairman Iain Vallance and Deputy Chairman Lord Marshall of Knightsbridge "The men who broke the bank at British Telecom.
Bonfield is presently a director of: AstraZeneca, Dubai International Capital, Sony Corporation, L.M. Ericsson, TSMC, NXP Semiconductors, and Mentor Graphics. He is also a non-executive director of the Department for Constitutional Affairs.
Bonfield was honored as a Commander of the Order of the British Empire in 1989, and subsequently dubbed a Knight Bachelor by Queen Elizabeth II in 1996. In 1995 he was awarded the Mountbatten Medal.
Married, he and his wife live in Richmond in a house by the River Thames.