Early career (2008–2011)

Walker spent his first year at the Crows playing for Norwood in the SANFL, kicking more than 50 goals in the 2008 season.^[5] He made his AFL debut in round 1 the next season, against Collingwood, and held his place in the side for the next 12 rounds.^[7] This included a five-goal performance against reigning premiers Hawthorn in round 10, which earned him an AFL Rising Star nomination.^[8] However, only three weeks later he was dropped from the senior side and played just one more match for the season, with coach Neil Craig wanting him to work on deficiencies in his game.^[7]

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 sets 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 incoming expansion team Greater Western Sydney, with whom Walker had been previously strongly linked.^[9] 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, at an average of 2.5 per game, ranked sixth in the AFL.^[10]

Improvement (2012–2014)

In 2012, Walker improved under new coach Brenton Sanderson. By Round 8, Walker led the Coleman Medal count before two separate suspensions for rough tackles cost him the chance to lead the league's goalscoring.^[11][12] However, Walker returned to finish the season in good form, booting five goals in Adelaide's come-from-behind Semi-Final win over Fremantle,^[13] and four goals the next week in the Crows' Preliminary Final loss to Hawthorn.^[14] In total, Walker kicked 63 goals for the season and averaged 3.3 goals per game, second in the AFL behind Lance Franklin.^[10]

In Round 5, 2013, against Carlton, Walker hyperextended his right knee and suffered an anterior cruciate ligament injury requiring a full reconstruction and 12-month rehabilitation period.^[15] 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 $66,672.45 for the foundation before having his head shaved by TV personality and former Geelong player Sam Newman.^[16]

Walker made his return to competitive football early in the 2014 season in the Adelaide Crows reserves' side, kicking three goals in a 37-point loss to Glenelg.^[17] He made his return to the AFL against Collingwood in Round 9 at the Adelaide Oval, and 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.^[10] In November, Walker extended his contract with the Crows for three further years, until the end of 2018.^[18]

Captaincy (2015–2019)

In January 2015, Walker was appointed captain of Adelaide.^[1] He returned to his best form in that year, finishing fourth in the Coleman Medal with 55 goals in the home-and-away season and leading the club inspirationally as they overcame the death of senior coach Phil Walsh to make the semi-finals.^[10] He played his 100th AFL game in round 18 against the Sydney Swans at the Sydney Cricket Ground, fittingly in his home state of New South Wales.^[19]

In 2016, he was voted the AFL Player's Association captain of the year.^[20]

Walker went into his third season as captain with a full pre-season of training,^[21] but when the season began there were concerns that a hamstring strain suffered in a JLT Series match against Brisbane Lions could rule him out of the Crows’ first match against Greater Western Sydney.^[22] He worked hard to recover, going to extra training sessions in the week leading up to the match,^[23] but was ultimately left out of the team. He returned in the second round against Hawthorn.^[24] Walker began the season with great form, starring with five goals and four goal assists in the first half against Essendon in the absence of fellow key forwards Josh Jenkins and Mitch McGovern.^[25] He kicked 18 goals in his first five games while Adelaide remained undefeated, but he began to struggle, only kicking 16 in his next seven with the Crows losing five of those games.^[26] Though he was still contracted to play for the club until 2018, Walker signed a contract extension to the end of 2021.^[27] In a match against the Western Bulldogs, Walker bumped defender Jason Johannisen into a goal post. Johannisen went on to play the rest of the match, but the incident was looked at by the Match Review Panel,^[28] who charged Walker with misconduct and fined him $1,000.^[29] Walker missed the final match of the home and away season against West Coast due to a toe injury.^[30]

Walker captained Adelaide in the 2017 AFL Grand Final, which they lost to Richmond. Walker performed underwhelmingly and admitted that he was one of the players who didn’t play their role well enough. After the match he was criticised over the brevity of his post-match speech.^[31] When teammate Jake Lever requested a trade from Adelaide to Melbourne, Walker was again criticised for saying that Lever was “choosing money over success”.^[32] Weeks later Bryce Gibbs was traded to Adelaide from Carlton and Walker was accused of holding double standards, which he denied saying that Gibbs had come to Adelaide to spend time with family and accepted a pay cut.^[33]

On 4 September 2019, Walker stepped down as the club's captain.^[34]

Post-captaincy (2020–present)

On 19 September 2020, Walker became the all-time leading goalkicker for the Adelaide Crows.^[4] He scored his 441st goal for the club in Round 18 of the shortened 2020 season against Richmond, surpassing Tony Modra's previous record of 440. He was also Adelaide's leading goalkicker for the COVID-shortened 2020 season, scoring 15 goals;^[35] this was the lowest tally for any club's highest scorer since Jack James totalled 13 goals for St Kilda in the 1920 season (exactly 100 years prior).^[36]

Walker started the 2021 season in resurgent form, kicking 17 goals in his first three games and beating his 2020 tally in the process, finishing the year with 48 goals, his highest return since 2017. He joined Jonathan Brown as the second AFL player since 1993 to kick five or more goals in each of the first three rounds of a season.^[37] He was widely lauded for returning to near career-best form in 2021, after a disappointing 2020 season where many commentators were questioning his future in the game.

Before the Round 21 Showdown match with Port Adelaide, news broke of Walker having made a racist comment about a player from an opposing team in the SANFL, Robbie Young. After an AFL investigation, Walker was suspended for six matches and fined $20,000.^[38] Walker made an apology video over the incident with Young in frame.^[39] Young accepted Walker's apology in the ABC interview; the apology which was criticised by some Indigenous leaders in the AFL community.^[40] Walker kicked four goals in his return game against Essendon in Round 4 of the 2022 season.

Walker had another outstanding season in 2022, being Adelaide's leading goalkicker for a record sixth time with 47 goals for the year, breaking the record set by Tony Modra in 1997. Some considered it to be better than his 2021 season. He kicked bags of five goals against Richmond in Round 5 and Collingwood in Round 18, as well as a bag of six against North Melbourne in Round 15. Walker was named in the All Australian squad of 40, but did not make the final team.

Walker had a somewhat quiet start to the 2023 season, kicking only three goals in the first three games. He returned to form in Round 4, kicking four goals against Fremantle. In his 250th match in Round 13, 2023, Walker would go on to kick a career-high 10 goals^[41] against an undermanned West Coast side,^[42] crushing his previous personal best of seven goals against the Brisbane Lions in 2015.^[43] In Showdown LIV in Round 20, Walker was awarded the Showdown Medal after a major seven goal haul.