Marcus Agius was born on 22 July 1946, the son of Ena Eleanora (née Hueffer) and Lieutenant Colonel Alfred Victor Louis Benedict Agius. He is of part Maltese descent, and was educated at St George's College, Weybridge. He earned his degree in Mechanical Sciences and Economics at Trinity Hall, Cambridge, and also holds an MBA from Harvard Business School.
Agius' banking career began at the investment bank, Lazard, where he worked from 1972 to 2006 holding several senior positions. He was appointed as the chairman of the London branch of Lazard in 2001 and deputy chairman of Lazard LLC in 2002. He joined BAA PLC as a non-executive director in 1995 and served as its chairman from 2002 till 2006.
On 1 September 2006, Agius joined the Barclays board as a non-executive director and succeeded Matthew Barrett as chairman from 1 January 2007. His most recently reported salary was £750,000.
On 2 July 2012 it was announced that Agius would resign following the Barclays fine for manipulation of the London Interbank Offered Rate (Libor) by some Barclays employees in the Libor scandal. In November 2012, it was reported that he might be retained as a consultant.
On 3 July 2012 it was announced that Agius was reinstated as "full-time chairman", following the resignation of CEO Bob Diamond. Agius will serve as chairman until a new chief executive is appointed. During his time at Barclays Agius attended the 2011 meeting of the Bilderberg Group.
Since 2010, Agius is the chairman of the British Bankers Association (BBA). Libor (formally BBA LIBOR) being calculated and published by Thomson Reuters on behalf of the BBA.
From December 2006, Agius is a non-executive director of the BBC's executive board. He is the chairman of the trustees of the Royal Botanic Gardens, Kew, and of the Kew Foundation. In January 2014, Agius was appointed non-executive chairman of PA Consulting. In January 2017, Agius was appointed the chairman of Reach South Academy Trust.
In September 2011, Agius was appointed as a committee member to act as one of the three trustees of the Bilderberg Group. The other committee members are Kenneth Clarke MP and Lord Kerr of Kinlochard.
Marcus, a Roman Catholic, married Katherine (born 1949), daughter of Edmund de Rothschild of the Rothschild banking family of England, and has a close involvement with the Rothschild family estate, Exbury Gardens in Hampshire. They have two children.
In 2010 The Tablet named him as one of Britain's most influential Roman Catholics. He is a keen gardener and art collector.