After turning professional in 1994 at the age of 18, Walker reached the quarter-finals of the World Championship in 1997, with victories over Dave Harold 10–7 and Alan McManus 13–10, before he lost 13–8 to Alain Robidoux. This was the first time he had reached the latter stages of a ranking tournament, and he also reached the last 16 of the same tournament in 2004 with a 10–7 win over Stephen Lee before losing 13–5 to David Gray. He dropped off the Main Tour after the 2005–2006 season, but returned a year later after a strong campaign on the Pontin’s International Open Series, from which the top 8 finishers gain Main Tour places. However he dropped off again at the end of the season.
Walker did however return to the tour in 2014 as he won a 2-year tour card by reaching the semi final stage of the second event of the 2014 Q School.
In 2016/2017, he had one of his most impressive seasons to date, the highlight being his run in his home tournament, the Welsh Open. He defeated Rhys Clark, and former world champions Neil Robertson and Graeme Dott before losing in the fourth round to Zhou Yuelong.