Brearley was educated at the City of London School (where his father Horace, himself a first-class cricketer, was a master). While at St. John's College, Cambridge, Brearley excelled at cricket (he was then a wicketkeeper/batsman). After making 76 on first-class debut as a wicketkeeper, he played for Cambridge University between 1961 and 1968 (captaining the side from 1964 onwards), first as an undergraduate on the moral sciences tripos, and then as a postgraduate. While still at Cambridge he was chosen for the MCC tour to South Africa in 1964—65, and to captain the MCC Under-25 side in Pakistan in 1966—67, when he scored 312 not out against North Zone (his highest first-class score) and 223 against the Pakistan Under-25 side he ended the tour with 793 runs from six matches at an average of 132.

From 1961 onwards he played for Middlesex County Cricket Club, often opening the innings with Michael Smith. As captain between 1971 and 1982, he led Middlesex to County Championships in 1976, 1977 (jointly with Kent), 1980 and 1982; and he appeared in Free Foresters' very last first-class fixture, in 1968, keeping wicket and scoring 91.

In part due to his pursuit of an academic career, which limited his cricketing activity in 1969 and 1970, Brearley was not selected for England until the age of 34 in 1976. Brearley's record in Test cricket at a batsman was mediocre (he averaged 22.88 in 66 Test innings, without a Test century), but he was an outstanding captain. Having previously kept wicket, he was also a very fine slip catcher, usually at first slip. He took over as captain of England in 1977. His excellent man-management skills (he was once described by Rodney Hogg as having "a degree in people") drew the very best from the players in his team, although he was fortunate to be able to call on the services of Bob Willis, David Gower and Ian Botham at their peak. Brearley was captain during the infamous aluminium bat incident in 1979, when he objected to Dennis Lillee's use of the bat, instead of one made of willow. On the same tour he caused controversy by ordering all his fielders including the wicketkeeper to the boundary with three runs required off the last ball of the match (this was legal by the rules of the time). He had been an innovator regarding cricket equipment himself, wearing a 'skull cap' under his England cap in 1977. It consisted of a plastic protector with two side pieces protecting his temples. It was later popularized by the Indian batsman Sunil Gavaskar.

Brearley also captained England to the final of the 1979 Cricket World Cup, scoring 53 in the semi-final against New Zealand and 64 in the final against the West Indies. However, his defensive opening partnership of 129 with Geoff Boycott in the final used up 38 of 60 allotted overs; although it was recognised that a potent pace attack of Andy Roberts, Michael Holding, Colin Croft and Joel Garner needed to be countered, the speed of the partnership greatly added to the pressure on the rest of the order. Garner bowled a spell of 5 wickets for 4 runs to induce a drastic collapse and hand the West Indies the match and the World Cup by 92 runs.

Having passed the England captaincy to Ian Botham in 1980 (losing his Test place in the process), Brearley returned as captain following Botham's resignation for the famous third Test against Australia at Headingley in 1981, famously going on to win the match and two of the remaining three matches of the series to win the Ashes 3-1. Brearley's extraordinary galvanising of Botham is regarded as one of the greatest feats of sporting psychology of all time: Botham recovered from personal ridicule due to his winless captaincy record and his nosedive in form (he had made a pair in the second Test at Lord's) to take a first-innings 6 for 95 and score 50 and his famous 149 not out in the third Test at Headingley, bowl a spell of 5 wickets for 1 run in the fourth Test at Edgbaston, score 118 from 102 balls in the fifth Test at Old Trafford, and take a 10-wicket match haul (6 for 125 and 4 for 128) in the sixth Test at the Oval.

Not all players have sung the praises of his captaincy with the same generosity as Ian Botham. Phil Edmonds, who played under Brearley at both county and country level, developed the practice of walking backwards to his bowling mark to ensure Brearley did not change the field behind his back. In his second autobiography, Fred Titmus, a senior county colleague, poured scorn on Brearley's reputed man-management skills.

Brearley is now a psychoanalyst, psychotherapist (registered with the BPC), motivational speaker, and part-time cricket journalist for The Observer. He was awarded the OBE in 1978, and published The Art of Captaincy in 1985. He opposed sporting links with apartheid South Africa, seconding a motion to the MCC in 1968 calling for the cessation of tours until there was actual progress towards non-racial cricket. He seconded the motion from David Sheppard to the MCC, calling for the England tour to South Africa to be cancelled, and was a supporter of John Arlott who campaigned in The Guardian for the same objective.

Brearley took over from Doug Insole as President of MCC on 1 October 2007, and chose Derek Underwood to succeed him at the end of his term. After the MCC dinner at Lord's at which he was appointed Brearley was seen at a bus stop waiting for his bus home...suggesting to some commentators that he might be more "unassuming" than his predecessors.