Downton was born at Farnborough in metropolitan Kent in 1957. He attended Sevenoaks Prep School, Sevenoaks School and the University of Exeter. He obtained a law degree, a coaching certificate and earned international honours at youth level in both cricket and rugby union.

His father, George, had played briefly for Kent in the post-war period and Downton's early county cricket career was with the same county. Kent shared the 1977 County Championship and won the completion outright in 1978 but Downton became frustrated deputising for Alan Knott in the 1979 season and moved to Middlesex in 1980.

On arrival, he has been described by a teammate there as "an intelligent, dapper individual ... who did not immediately seem to fit with the rather cruder, laddish Londoners in the team". On his first-class debut with Middlesex, the captain, Mike Brearley, chose to use Downton as an opener, and the two registered an opening partnership of 160.

Downton tasted Test cricket for the first time in the West Indies that winter. He was dropped after the first Test against Australia in the summer of 1981 and had to wait until the summer of 1984 for further such honours, when he was picked at home to bolster England's batting options against the West Indies. It began an uninterrupted twenty-three match run in the national team.

A trustworthy, affable and resourceful team member, Downton played a starring role in Middlesex's successes in the 1980s. The cricket journalist and Downton's former Middlesex teammate Simon Hughes has described Downton as "gracious, encouraging, unfailingly polite, a diplomat. The kind of chap ... who would offer the bailiffs a glass of sherry if they turned up unannounced." Hughes argues that Downton's calm personality, even when dealing with a difficult colleague, was a significant factor in Downton's success as a player and stand-in captain at Middlesex while Brearley was absent playing for England.

Downton's cricket career ended in 1991 when he failed to recover from a freak eye injury. A bail lodged in his eye when he was standing up to the stumps during a Sunday League match at Basingstoke in 1990.^[1]

After retiring from cricket, he became a stockbroker with James Capel and Cazenove & Co, having been "partly immersed in the City"^[2] by the time of his accident.

On 16 October 2013, Downton was appointed to the position of Managing Director of the England and Wales Cricket Board with effect from 1 February 2014. He replaced Hugh Morris who resigned to become chief executive and managing director of Glamorgan County Cricket Club.^[3] He left the role on 8 April 2015.^[4]

Kent announced his appointment as Director of Cricket on 18 January 2018.