Martyn moved with his family to Perth, Western Australia at the age of three, and was educated at Girrawheen Senior High School, before being selected for the Australian Cricket Academy in Adelaide in 1990. He made his first class debut for Western Australia the following season, in which he scored 822 runs at 51.37.

Noted for his daring and sometimes cavalier strokeplay, Martyn was selected to make his Test debut against the West Indies in November, 1992 at the Gabba, a surprise replacement for Dean Jones. He batted in the middle-order, scoring 36 and 15. He played in all Tests in that season, apart from the Fourth Test at the Adelaide Oval where he was replaced by fellow Western Australian Justin Langer after sustaining an injury before the match.

Martyn was subsequently dropped from the Test team in the early 1993 tour to New Zealand, until being recalled for the Third Test at Eden Park, Auckland after Mark Waugh was dropped, scoring 74. Martyn also played in some of the subsequent ODIs on the New Zealand tour as well as on the 1993 Ashes tour to England, but was dropped from the Test team after Waugh scored centuries in the warm-up matches and had been the leading run scorer in the New Zealand ODIs. Waugh was third in the Test batting averages, confining Martyn to playing in tour matches against county teams. Martyn was given another chance in the 1993–94 Australian season when Steve Waugh tore a hamstring and Martyn played in the Boxing Day and New Year’s Tests at the MCG and SCG respectively. Although he managed 59 in the first innings of a low scoring match in Sydney, it was his performance in the second innings which blighted his reputation for many years into the future. With Australia stumbling while chasing a small target of 117 against South Africa, Martyn had trouble scoring, with an unconvincing 6 from 59 balls in almost two hours as he and Craig McDermott took the total from 8–75 to within sight of victory at 8–110. He then attempted an airborne cover-drive off Allan Donald and was caught, and Australia lost the Test. Martyn was heavily criticised by the media who blamed him for the defeat. He played three more ODIs in that season, before being dropped upon Waugh’s return from injury. Although Waugh would have reprised his position in any case, Martyn lost his position as the reserve batsman and was dropped from subsequent national squads altogether. He would not play ODI cricket again until 1997 and Tests until 2000.

He re-appeared at the start of the millennium when he replaced an injured Ricky Ponting on the tour to New Zealand in early 2000. Despite topping the averages in New Zealand he made way for Ponting for the home series against the West Indies in 2000/01. He played one test during that summer, replacing an injured Steve Waugh for Australia’s victory at Adelaide where he remained not out in both innings.

Martyn’s first ODI century came against New Zealand on 3 March 2000 at Eden Park, Auckland, he scored 116 and remained not out. A knock of 144 runs from 149 balls against Zimbabwe on 4 February 2001 at the WACA Ground, Perth is his highest ODI score. His last ever ODI century was against India on 1 November 2003 at Wankhede Stadium, Mumbai where he scored 100 runs and also won the man of the match award.

He was selected on Australia’s tour to India in 2001 and was considered unlucky not to replace a struggling Ponting. His chance to consolidate a permanent position in the side came during the 2001 Ashes tour where he replaced fellow West Australian Justin Langer in the first test. He posted his maiden test century in the first test at Edgbaston. He was chosen as a Wisden Cricketer of the Year as a result of his performances in The Ashes in the summer of 2001.

Martyn scored his first century, about 9 years after making his debut, in The Ashes, against England on 5 July 2001 at Edgbaston Cricket Ground, Birmingham scoring 105 runs in the process. His performance in the series earned him the Wisden cricketer of the year award for 2001. His highest score of 165 runs came against New Zealand on 18 March 2005 at Basin Reserve, Wellington. His last ever Test century was against South Africa on 31 March 2006 at The Wanderers Stadium, Johannesburg where he scored 101 runs. Out of his 13 Test centuries, 8 has been scored on the first innings and the rest 5 in the second innings.

Martyn subsequently put in regular appearances for Australia, becoming integral to the middle-order and eventually filling the void at no. 4 left by Mark Waugh’s international retirement. His best form came over a twelve-month period beginning in March 2004 where he scored two crucial centuries to help Australia to a 3–0 series whitewash in Sri Lanka. He was then man of the series in India when Australia claimed their first series victory on Indian soil in over 30 years. He made two centuries and narrowly missed out on becoming the first Australian batsman since Don Bradman to make three centuries in three innings when he was dismissed for 97 in the second innings of the third test. Two centuries at home against Pakistan over the 2004/05 season and another in New Zealand in 2005 rounded out a stellar 12 months in which Martyn had made 1608 runs at an average of 61.84 in 12 months including 7 test centuries.

Martyn was presented with the McGilvray Medal in 2005 as the ABC’s Test cricketer of the year as well as being adjudged Test Player of the Year at the 2005 Allan Border Medal.

Following a poor Ashes tour of England in 2005, where he recorded a total of 178 runs at an average of 19.77, Martyn was dropped from the Australian Test side to play in the ICC Super Series 2005. However, he was retained in the ODI side over the 2005/06 summer.

In the 2006 Champions Trophy Tournament, Martyn won back-to-back man of the match awards against England and India. He also played an important unbeaten innings in the final against the West Indies to help Australia win the tournament for the first time. He was Australia’s leading runscorer in the tournament.

Despite his poor form Martyn was widely expected to retain his place due to Shane Watson’s lack of fitness and the fact that the third test was to be played at the W.A.C.A. Ground, Martyn’s home ground. It came as a surprise when on 8 December 2006, Martyn announced his immediate retirement from all domestic and international cricket. Commenting on his decision, Martyn said: “I would like to advise of my retirement from cricket, effective from today. I do so with a deep awareness of the opportunities that the game and Cricket Australia have provided for me.” Cricket Australia CEO James Sutherland insisted that Martyn had not been pushed and that Martyn was originally going to be named in the squad for the third Test.

On 1 September 2007, it was reported in the Daily Telegraph that Martyn had joined the break-away Indian Cricket League. However, since then he has pulled out of the event due to the birth of his first son, Ryder, but rejoined the ICL for 2008.

On 19 January 2010, Martyn was signed for US$100,000 to play for the Rajasthan Royals in the Indian Premier League.