Walker attended Willyama High School in Broken Hill. His father Wayne, was a prominent footballer in Broken Hill, and was known for his aggressive attack on the ball, which often left several of his opponents in his wake. In 2006, Walker accepted a NSW Scholarship contract with Adelaide, at the age of 16. However, he continued to play for his senior local amateur league team, North Broken Hill, in the Broken Hill Football League, leading them to the 2007 Premiership with a seven-goal, best on ground performance, playing at centre half forward.
In 2008, after being drafted to the Adelaide's senior list, he began to play for Norwood in the SANFL. Walker kicked 56 goals in his first year in the SANFL, including seven goals in an elimination final victory over rivals Port Adelaide.
Walker made his AFL debut against Collingwood in round 1 of his second season, in 2009. He instantly impressed, showing signs of forming a lethal forward combination with fellow Adelaide young forward Kurt Tippett, and received an AFL Rising Star nomination following a five-goal performance against Hawthorn in Round 10. However, he was dropped from the senior side later in the season, on the back of a few poor performances where his defensive pressure was particularly criticised.
Walker returned to the side in 2010 and showed fluctuating form, showing glimpses of brilliance alongside some poor performances. He was dropped from the side on a number of occasions throughout the year, sparking rumours that he and head coach Neil Craig were not seeing eye to eye and that Craig was using Walker as a scapegoat for Adelaide's poor form throughout the year. Again, Walker was accused of not showing enough defensive pressure in the forward line. He finished with 35 goals for the season from 18 games, including two bags of 4 goals against Fremantle and then Geelong.
Early in 2011, Walker was dropped once again and was seen drinking a beer on live television at a SANFL game, sparking speculation that he 'didn't care' and may have signed a contract with Greater Western Sydney, the eighteenth side in the AFL who entered the competition in 2012 and with whom Walker had been previously strongly linked. However, late in the season, Walker re-signed with the Crows for a further four years. Despite inconsistent form and injuries, Walker was the club's leading goalkicker in 2011, booting 32 goals from only 13 games, and was ranked sixth in the AFL for goals kicked per game.
2012 saw Walker improve under new coach Brenton Sanderson. As of round 8, Walker led the Coleman Medal count. Suspensions for two rough tackles cost him the chance to lead the league's goalscoring. However, Walker returned to finish the season in good form, kicking 4 goals in each game from rounds 19 to 22. Walker shone in the Crows' finals campaign, booting five goals in a best-on-ground display as Adelaide came from behind to defeat Fremantle in the Semi Final. He then kicked four goals the next week in the Crows' Preliminary Final loss to Hawthorn. In total, Walker kicked 63 goals for the season, the most by any Crow since Tony Modra in 1997.
In round 5, 2013, against Carlton, Walker badly hyper-extended his right knee and suffered an ACL injury, requiring a full reconstruction and 12-month rehabilitation period. In the meantime, Walker made headlines by appearing on The Footy Show and agreeing to shave his famous mullet to raise money for the Leukaemia Foundation. He then went on to raise a total of $66,672 for the foundation before having his head shaved by TV personality and former Geelong star Sam Newman.
Walker made his return to competitive football early in the 2014 season in the Adelaide Crows reserves' side with games against Glenelg and Port Adelaide. He made his return to the AFL against Collingwood in Round 9 at the Adelaide Oval, failing to kick a goal, but truly announced his return against Gold Coast two weeks later, booting five goals and gathering 22 disposals in a best on ground performance. Walker played every game for the remainder of the season, kicking multiple goals in 11 games and a career-best six against the Brisbane Lions. He finished with 34 goals in 15 games, and averaged a career-high 14.7 disposals per game.
On 14 January 2015, Walker was appointed captain of Adelaide.