Rookie year (2013)

On 20 April, Walker played for the New South Wales Under 20s team against the Queensland Under 20s team at centre in the Blues 36–12 win at Penrith Stadium.^[5][6] In Round 10, Walker made his NRL debut for the South Sydney Rabbitohs against the Wests Tigers at centre and scoring a try in South Sydney's 54–10 win at ANZ Stadium.^[7] Walker finished his promising debut year in the NRL with him playing in 16 matches, scoring 8 tries and kicking 1 goal.

Representative debut season, & the end of South Sydney’s 47-year drought (2014)

On 14 February, Walker was named in Souths inaugural 2014 Auckland Nines squad.^[8] At the start of the 2014 NRL season, due to the new Souths five-eighth Luke Keary getting injured in the Nines tournament and John Sutton being shifted to play in the forwards, Walker was appointed to be five-eighth.^[9] Walker played five-eighth in his junior years.

He had his first start at five-eighth in round 1 against the Sydney Roosters where the South Sydney club won 28–8.^[10] Walker played at five-eighth till round 4, being shifted back to his preferred position at centre and John Sutton filling in at five-eighth until Keary returned from injury. On 4 May, Walker was selected to play at centre for City Origin against Country Origin in the 26–26 all draw in Dubbo.^[11][12] In Round 9 against the Gold Coast Titans, Walker scored 4 tries in South Sydney's 40–18 win.^[13]

Walker finished a breakout season playing in 24 games, scoring 12 tries and kicking 1 goal as Souths finished 2nd on the ladder. Walker scored a try in South’s 40-24 qualifying final victory against Manly. Walker set up a try in Souths’ 32-22 preliminary final win against the Sydney Roosters. On 5 October, 2014 NRL Grand Final, Walker played at centre in South Sydney's 30–6 victory against the Canterbury-Bankstown Bulldogs.^[14][15]

On 14 October 2014, Walker was selected for the Australian Kangaroos 2014 Four Nations 24-man squad.^[16] Walker made his international debut against New Zealand at Suncorp Stadium, starting at centre then played at fullback in the 2nd half filling in for Greg Inglis in Australia's 30–12 loss.^[17] Walker played in all of Australia's 4 matches, including playing at centre in the 22–18 loss in the final against New Zealand at Westpac Stadium.^[18]

New South Wales debut overshadowed by off-field incidents, and a fresh start (2015)

On 23 January 2015, Walker was named in the South Sydney Rabbitohs tournament winning 2015 NRL Auckland Nines squad.^[19] On 13 February, Walker played centre for NRL All Stars for the 2015 All Stars match at Cbus Super Stadium. Walker scored a try in the side's 20–6 loss to the Indigenous All Stars.^[20] On 23 February 2015, Walker played for the Rabbitohs in the 2015 World Club Series match against 2014 Super League Grand Final premiers St. Helens, playing at centre and scored a try in South Sydney's 39–0 win at Langtree Park.^[21]

In the 2015 NRL season Round 1 opener against the Brisbane Broncos at Suncorp Stadium, Walker scored the first try of the 2015 season in South Sydney's 36–6 win.^[22] On 13 June 2015, Walker was included into the New South Wales Blues origin squad as 19th man leading up to Game 2 of the 2015 State of Origin series.^[23] Walker finished the 2015 NRL season with him playing in 22 matches and scoring 13 tries as South Sydney finished in 7th position on the ladder. Walker scored a try in Souths’ 26-12 loss to the Cronulla-Sutherland Sharks in Week 1 of the 2015 NRL finals.

In the early hours of 22 September 2015, Walker and fellow South Sydney teammate and best friend Aaron Gray were hospitalized after the pair overdosed on prescription drugs Oxycodone and Tramadol. The pair were discovered by another friend of them who alerted emergency services and the pair were sent to St Vincent's Hospital in a critical condition placed in induced comas.^[24] The pair were released from hospital three days later to front the media about the unfortunate situation.^[25][26] Later On 26 November, Walker was released from the final year of his Rabbitohs contract after requesting a release, after finding out that he was fined five percent of his contract and with the club unhappy with him for bringing it into disrepute.^[27][28][29]

On 17 December 2015, Walker signed with the Manly Warringah Sea Eagles on a four-year deal starting from 2016, after he was heavily linked to sign with them since his release from the Rabbitohs, putting the end to his tumultuous off season.

Although he had played most of his first-grade career in the centres, Manly coach Trent Barrett and Manly-Warringah legend and rugby league Immortal Bob Fulton, named Walker as the replacement five-eighth for New Zealand international half Kieran Foran, who left Manly for Parramatta. At the official announcement of his signing during a Manly pre-season training camp in Coffs Harbour, Walker told the Sydney Morning Herald "Five-eighth was my favourite position coming through the juniors and if the coach (Barrett) wants me there I will jump at the opportunity".^[30][31]

Shift from centre to five-eighth, New South Wales representative debut (2016)

On 13 January, Walker was named in the emerging New South Wales Blues squad.^[32] In Round 1 of the 2016 NRL season, he made his club debut for Manly-Warringah against the Canterbury-Bankstown Bulldogs, starting at five-eighth in the Sea Eagles' 28–6 loss at Brookvale Oval.^[33] In Round 2 against the Wests Tigers, he scored his first club try for Manly in the 36–22 loss at Leichhardt Oval.^[34]

On 1 June, he made his debut for New South Wales against Queensland in Game 1 of the 2016 State of Origin series, coming off the interchange bench. He came onto the field at the 71st minute, and made an error shortly after.^[35][36]

In July 2016, Walker was fined $10,000 by Manly-Warringah after he punched his front door and injured his hand. Walker and his partner reported that he had punched the door out of frustration over an undisclosed incident. Walker was ruled out for a month as his hand required surgery.^[37]

Walker finished the 2016 NRL season playing in 18 games, scoring 8 tries and kicking 12 goals as Manly finished 13th on the ladder, which meant Walker would not qualify for the finals for the first time in his career.

Dally M Centre Of The Year (2017)

For the 2017 NRL season, Walker was shifted back to centre in the Manly side. In round 8 against Canberra, Walker missed a conversion to win the game for Manly in normal time and got into an on-field incident with Canberra player Josh Hodgson.^{[citation needed]} In extra time, Walker kicked a penalty goal to win the game for Manly despite being heckled by the Canberra players behind the goal. On 28 June 2017 against Cronulla, Cronulla player Jayden Brailey attempted to tackle Walker but broke his jaw in two places as a result, Walker was seen on camera abusing Brailey as he left the field.

At the end of the 2017 NRL season, Walker was named Dally M Centre of the year, scoring 13 tries in 13 games, and kicking 28 goals as Manly finished the season in 6th position. Walker played at centre in Manly’s loss in Week 1 of the finals to the Penrith Panthers. On 28 September 2017, Walker broke his leg and fractured his ankle while playing for The Australian Prime Ministers’ XIII side.^[38][39][40]

Manly narrowly avoid the wooden spoon (2018)

In Round 11, Walker suffered a fractured eye socket after getting into a fist fight with Melbourne player Curtis Scott. According to the other players on the field, Walker had been heckling Scott for most of the game when the fight broke out in the second half. After the game scans revealed that Walker would be out for 6–8 weeks.^[41][42]

Walker finished the 2018 NRL season playing in 14 games (4 games at centre and 10 games at five-eighth) and scoring 2 tries as Manly failed to qualify for the finals, finishing in 15th position.

Another solid season en route to the finals (2019)

In round 10 of the 2019 NRL season, Walker played his first game of the season at centre and scored a try in his first game back from his suspension from the NRL due to an off-field incident. In Round 18, Walker shifted positions from centre to five-eighth, where he played for the rest of the year.

Walker made a total of 17 appearances for Manly in 2019 as the club finished 6th on the table and qualified for the finals. Walker played in both finals matches as the club reached the elimination semi-final against South Sydney in which Manly were defeated by his old club 34–26 at ANZ Stadium.^[43][44][45]

Māori All Stars debut and a mediocre season for Manly (2020)

On 22 February, Walker made his debut for the Māori All Stars and played at centre in their 30-16 victory over the Indigenous All Stars.

Walker played a total of seven games at five-eighth for Manly-Warringah in the 2020 NRL season as they finished a disappointing 13th on the table.^[46]

First glimpses of the ability to play different position's, and Manly’s best season finish in years (2021)

In Round 1, Walker started the 2021 NRL season at fullback where he played the first 5 games, before reverting back to his preferred position of centre, after Dally M Player of the Year, Tom Trbojevic, returned to the squad from injury. He also played a game at halfback. In Round 14, he came off the interchange bench for the first time in his first-grade career where he was able to come on the field as an impact player.

Walker finished the season playing 20 games for Manly, and scoring 5 tries. Walker played at centre in the club's preliminary final loss against South Sydney.^[47]

Venturing into the forward pack, adapting into a specialist utility (2022)

On 12 February, Walker played at centre in the Māori All Stars 16-10 victory in the 2022 All Stars Match. On 2 March, Walker signed a three-year deal to join the New Zealand Warriors starting in 2023.^[48]

Coming off the interchange bench for majority of games during the season, Walker played as a hooker and also a lock, adding to his repertoire of positions Walker can play, fully developing into a specialist utility. Walker played a total of 19 games for Manly in the 2022 NRL season as the club finished 11th on the table and missed the finals. Walker's final game for Manly was a 21–20 loss to Canterbury in round 25 of the competition.^[49]

Return to career best form as a forward, and the ultimate utility (2023)

Walker made his club debut for New Zealand in round 1 of the 2023 NRL season against Newcastle.

Walker finished the 2023 NRL season playing in 24 games in his best season in many years. Walker’s veteran experience and his versatility and ability to play multiple positions (prop, hooker, second row, lock, centre or five-eighth) helped guide the New Zealand Warriors to their first top 4 ladder finish since 2008.

Due to a calf injury to starting halfback Shaun Johnson before Week 1 of the Finals, Walker was shifted to five-eighth in the Warriors’ 32-6 loss to Penrith. In Week 2, Walker came off the interchange bench and scored a try in the Warriors’ victory against the Newcastle Knights. Walker played off the bench and put in a strong performance in the Warriors’ preliminary final loss to the Brisbane Broncos.^[50]