Mallett moved to Rhodesia with his family in 1956 when he was only six weeks old. He first arrived in Cape Town, South Africa in 1963 where he later graduated from the University of Cape Town in 1977. While a student at the univesity, he was selected to play for the Western Province rugby union team.
In 1979 Mallett moved back to England to attend Oxford University where he not only gained further qualifications but also won Blues in rugby union and cricket. Eventually he returned to South Africa, where he represented Western Province in four consecutive Currie Cup wins between 1982 and 1985, and played two games for the Springboks in 1984.
Mallett once again left South Africa in 1985, this time for France, where he played and coached rugby for seven years until 1992 before eventually returning to South Africa in 1994 and taking a job as Head of the False Bay Rugby Club until 1995.
Between August 1997 and December 1998, under Mallett's guidance, the Springboks went on a record winning streak of 17 consecutive test wins. As part of the unbeaten run the Springboks won the Tri Nations Series undefeated and beat several teams by record margins, including a 61-22 demolition of Australia, 52-10 humiliation of France in Paris, 68-10 win over Scotland in Edinburgh, 33-0 defeat of Ireland and 96-13 drubbing of Wales. The run ended when an exhausted Springbok team was defeated by England at Twickenham at the end of a long tour on the 5th of December 1998.
The relationship between Mallet and Gary Teichmann, arguably South Africa's most successful captain ever, began to sour and Teichman was controversially excluded from the 1999 Rugby Union World Cup squad. Mallet desperately looked for a new captain, first turning to Corné Krige then Rassie Erasmus, Joost van der Westhuizen and Andre Vos for a solution. In the end the internal instability in the squad seemed to harm their performance as the squad suffered 4 consecutive defeats and were finally knocked out of the championship in the semi-final by eventual winners Australia. Despite his teams poor shape in 1999 they still managed to break more records, beating Italy 101-0 and England 44-21, with Jannie de Beer kicking a world record 5 dropgoals in the latter test to smash England's World Cup dreams.
In 2000, Mallett accused the South Africa Rugby Football Union (SARFU) of "greed" for selling Tri-Nations championship tickets at inflated prices. He had alientated the SARFU executive, and on September 27, he resigned as national coach at the start of a disciplinary hearing began into allegations that his comments had brought the game into disrepute. Some fans, upset by how he had treated Teichman and his teams sudden poor performance, were also keen to see him go.
Mallett moved back to France as coach for the Paris club Stade Français, which he led to two consecutive French domestic title wins in 2003 and 2004 before returning to South Africa where he accepted the job of Director of Rugby in the Western Province. Initially there was speculation that he might coach the Springbok team again, but those rumours were quashed by the appointment of Jake White as the new South African coach.
In spite of his team's poor performance and the internal strife that characterised his final years as coach, Mallett remains one of South Africa's most successful coaches ever, having won 27 of the 38 tests played under his guidance and rewriting the record books several times.