Robertson began his snooker career at 14, when he became the youngest player to make a century break in an Australian ranking event. He began his professional career in the 1998/1999 season. Then, when he was 17 years old, he reached the third qualifying round of the 1999 World Championship.

In July 2003, Robertson won the World Under-21 Snooker Championship in New Zealand.This earned him a vital wildcard spot on the subsequent WPBSA Main Tour. In 2003 he won the qualifying tournament for a wildcard place at the 2004 Masters, where he subsequently lost 2–6 to Jimmy White in the first round.

In 2004/2005 season, he moved up to the top 32 in the rankings, reaching the final stages of 6 of the 8 tournaments, despite having to play at least 2 qualifying matches for each one. He qualified for the final stages of the 2005 World Championship, losing 7–10 to Stephen Hendry in the first round.

In the 2005/2006 season, he continued to progress, moving up to the top 16 of the rankings at the end of the season. He reached 4 quarter-finals in the season, including the 2006 World Championships, in which he fought back from 8–12 down to level at 12–12 against eventual champion Graeme Dott, before losing the final frame by inadvertently potting the final pink, which he needed on the table in his attempts to snooker the Scotsman.

In October 2009, Robertson clinched the 2009 Grand Prix trophy in Glasgow with a 9–4 win over Chinas Ding Junhui in the final. His semi-final match with defending champion John Higgins was won on the final black of the deciding frame. Robertsons fourth title made him the most successful player from outside the UK and Ireland in ranking tournaments, although Ding equalled his total at that seasons UK Championship. He achieved his 100th career century during the 2009 Grand Prix.

On 1 April 2010 Robertson made the first official maximum break of his career in his second round match in the 2010 China Open against Peter Ebdon.

At the 2010 World Championship, Robertson defeated Fergal OBrien 10–5 in the first round. In his second round match against Martin Gould Robertson trailed 0–6 and 5–11 before recovering to win the match 13–12. In the quarter-finals he defeated Steve Davis 13–5. He faced Ali Carter in the semi-finals, winning 17–12 to reach the final. There he defeated 2006 champion Graeme Dott 18–13 to become only the third player from outside the UK (and only the second from outside the UK and Ireland), and the first Australian, to become world champion in the modern era of the game. The win took him to a career-high ranking of No. 2 in the following season. Although the record books show Australian Horace Lindrum triumphed in 1952, that was the year when the sports leading players staged a boycott and to this day in many circles Lindrum is not regarded as a credible world champion.