Debut: v Celtic Crusaders 18.04.08

Leeds Honours: Grand Final (Winner: 2012, 2015, 2017), Challenge Cup (Winner: 2014, 2015; Finalist: 2011, 2012), League Leaders Shield (Winner: 2009, 2015), World Club Challenge (Winner: 2012; Finalist: 2009, 2010, 2013, 2016, 2018)

Kallum joined the Rhinos in 2007, having impressed during a trial period with the club.  In his first season with the Junior Academy, he scored 13 tries in 20 appearances as they headed for Grand Final glory, when he scored the opening try in the Rhinos' 26-16 win over St Helens.

That year he also earned himself a place in the England U17 squad to play at the Australian Institute of Sport. He began 2008 in spectacular form for the Academy, scoring 11 tries in the opening five games, including a hat-trick in the opening game for the Reserve Academy against Hull KR and four tries a week later against Castleford. This led to him being promoted to the Rhinos first team squad in March 2008, when he was still only 16 years of age.

His first team debut came against Celtic Crusaders in the Challenge Cup, and he helped the team to victory with a try. He was then named in the side to take on Bradford in the Millennium Magic and once again showed his attacking flair with the first try of the game.

In 2009, he was a regular in the first team squad, making 14 appearances during the campaign and scoring four tries, including a spectacular length of the field interception try at Headingley to beat Huddersfield Giants.

The 2010 season was frustrating for the former Latchford Albion player as he was sidelined for the majority of the campaign after he was forced to have a knee reconstruction after only the seventh game of the season.

It was a bitter blow for Kallum, after making a great start to the season, where he had proved a valuable player with his ability to play at centre, full back and wing, where he started the World Club Challenge game against Melbourne Storm. He fought back from this setback to make 17 appearances in 2011, which included a place in the Challenge Cup Final, and the following year he was handed the number 3 shirt ahead of the 2012 season, taking over from Keith Senior.

In 2012, Kallum finally got his hands on silverware as part of both the World Club and Grand Final winning sides.  He was in outstanding form in 2013 as the club’s top try scorer with 20, and this included a run of scoring in eight consecutive games.

He added the Challenge Cup to his list of honours in 2014, before an outstanding 2015 campaign that saw him win Rhinos Player of the Season after playing a key role in the treble-winning season. In 2016 he played in every minute of every game for the Rhinos and finished the season as the club’s top ball carrier (398), making the most metres for a back (2,800) and was the Rhinos' top try scorer with 18.

The following year, he once again proved to play a key role in the team's success, as he was one of the Rhinos' most consistent players and won his third Super League winners ring as part of the team that triumphed over Castleford in the Grand Final.

In 2018 he was named as the successor to Danny McGuire as captain of the club but after 16 appearances in the 2018 season, which included captaining the team in the World Club Challenge against Melbourne at the AAMI Stadium, a knee injury ruled the centre out for the remainder of the season. He returned to action in 2019, which was also his testimonial year, but with the Rhinos struggling from the start of the season under Dave Furner, he agreed to join NRL side Gold Coast Titans. The Rhinos had already confirmed that the centre would leave the club at the end of the current season, but the opportunity to join the Titans on a long-term contract resulted in the club agreeing to release Watkins to move to Australia at the end of July. He returned to Super League in 2020, signing with Salford Red Devils, and played against his former side in the Challenge Cup Final.

After a spell at the Red Devils, Watkins returned to Leeds midway through the 2025 season, joining on a deal until the end of the campaign. The move proved to be a masterstroke for all parties as Watkins rediscovered his former Leeds form at loose forward and second row as he made 21 appearances and scored four tries. His form earned Watkins a new deal in June 2025 as he agreed an extension until the end of the 2026 season. Even at 35 years old, his ability to remain on the field saw Watkins play every game in the forward's first full season back in Rhinos colours, starting in the second row before moving to operate as the side's loose forward for the bulk of the campaign. Scoring four tries in games against Wakefield, Bradford, York and Hull FC, he started at second row against Warrington in the club's semi-final outing against Warrington and renewed his contract for another year to feature in 2027. 

Kallum made his debut for England U17 against the Australian Institute of Sport as a substitute in the first test at Warrington's Halliwell Jones Stadium. England were unlucky losers that night, 28-26, having led for the majority of the game; however, a week later Kallum helped the young Lions restore pride with a 38-22 win at Wakefield's Belle Vue.

He made his England debut against Wales and was part of the squad that won the Autumn International series, during which he was one of the standout players in the side and was named Man of the Match after scoring a hat-trick in the win against France.

He has been a regular for the England squad since 2012, featuring in five of their 2013 Rugby League World Cup games and was selected for the squad for the Four Nations in 2014 and 2016.

In 2017, he was selected, alongside team mate Ryan Hall, for the England World Cup squad in Australia and was named Man of the Match in their group game against Lebanon.  He came agonisingly close to helping England to World Cup glory against Australia, where they just missed out 6-0 against the host nation.