At 15 years of age, Klemmer signed with the
Canterbury-Bankstown Bulldogs. In 2010 and 2011, Klemmer played for the Australian Schoolboys. In 2011, Klemmer also played for the New South Wales under 18s team. He played for Canterbury-Bankstown's
NYC team in 2011 and 2012, scoring 6 tries in 28 games. In 2012, Klemmer was named on the interchange bench for
New South Wales in the inaugural under 20s State of Origin match at
Penrith Stadium. At the end of 2012, Klemmer was named NYC Player of the Year and also named at starting in the 2012 NYC Team of the Year. In round 1 of the
2013 NRL season, Klemmer made his
NRL debut for Canterbury-Bankstown against the
North Queensland Cowboys off the interchange bench in Canterbury's 12–24 loss at
Central Coast Stadium. In round 3, against the
Melbourne Storm at
AAMI Park, Klemmer made headlines after he was at the end of receiving a high flying karate style kick from Melbourne's fullback
Billy Slater to the head in Canterbury's 18–22 loss. On 20 April 2013, Klemmer was named captain of the New South Wales team in the under 20s State of Origin match, leading the team to a 36–12 victory over Queensland at Penrith Stadium. Klemmer debut season ended early after he sustained a knee injury. Klemmer finished his debut year with him playing 4 matches for
Canterbury-Bankstown.
2014 In round 4 against the Melbourne Storm, Klemmer scored his first NRL career try in the Canterbury club's 40–12 win at
nib Stadium in
Perth. On 26 April 2014, Klemmer was selected to play for City Origin for the
City vs Country Origin match at Apex Oval in
Dubbo. Klemmer played off the interchange bench in the 26-all draw on 4 May 2014. On 5 October 2014, in Canterbury's
2014 NRL Grand Final against the
South Sydney Rabbitohs, Klemmer played off the bench in the club's 6–30 loss. Klemmer finished the Canterbury club's
2014 season with him playing 23 matches and scoring a try. Klemmer made his international debut for
Australia, playing against
England at
AAMI Park in Australia's 16–12 win.
2015 In round 5 of the
2015 NRL season, in the Grand Final re-match against the
South Sydney Rabbitohs, Klemmer was sin binned after crowding in on referee
Gerard Sutton alongside Canterbury captain James Graham and yelling "You're off your f***ing face". The Canterbury club were leading 17–16 at the time, but lost 17–18 at
ANZ Stadium. After full-time was blown, some members of the Canterbury fan base pelted referee Gerard Sutton and match officials with plastic bottles, causing an investigation to find out and ban the misbehaving members from attending rugby league matches for life. Klemmer was later hit with a three-week suspension for contrary conduct. He returned in round 9, against the
North Queensland Cowboys, in Canterbury's 23–16 loss at
1300SMILES Stadium. On 3 May, Klemmer played for
New South Wales City, playing at prop in the 34–22 loss to
New South Wales Country at
Wagga Wagga. On 27 May, he was selected to make his State of Origin debut for
New South Wales in game 1 against
Queensland. He played of the interchange bench in the Blues' 11–10 loss at ANZ Stadium. In game 2 of the
2015 State of Origin series, he again made headlines after a heated exchange with the oldest member of the Queensland team,
Corey Parker. Parker told Klemmer (the youngest member of the New South Wales team) to "show some respect" and Klemmer replied "get f***d you c**t". Klemmer also played in game 3, off the interchange bench, in the Blues' 52–6 record loss at
Suncorp Stadium. He finished off the 2015 season having played in 21 matches and scoring a try for Canterbury-Bankstown.
2016 On 22 May 2016, Klemmer extended his contract with Canterbury-Bankstown until the end of the 2020 season. Klemmer played in all 3 matches for
New South Wales off the interchange bench in the Blues 2–1 series loss in the
2016 State of Origin series. After Game 2, Klemmer got into some trouble after the Blues 26–16 loss to Queensland after he punched a wall in frustration in the toilets off the
Suncorp Stadium changing rooms. Klemmer finished the
2016 NRL season with him playing in 22 matches and scoring a try for Canterbury-Bankstown. On 24 September 2016, Klemmer played for
Prime Minister's XIII against
Papua New Guinea, playing off the interchange bench in the 58–0 win at
Port Moresby. On 3 October 2016, Klemmer was selected in the 24-man Australian Kangaroos squad for the
2016 Four Nations. Klemmer played in all 5 matches of the tournament, including playing off the interchange bench in the Kangaroos 34-8 Four Nations final win against
New Zealand at
Anfield.
2017 In February, Klemmer was selected in Canterbury's
2017 NRL Auckland Nines squad. On 5 May, he played for Australia in the
2017 ANZAC Test where he started at prop in the 30–12 win at
Canberra Stadium. In June 2017, it was rumoured that Klemmer requested a release from his contract from Canterbury-Bankstown due to a financial dispute stemming from his decision to split with long-time manager David Riolo. Klemmer later commented saying that he never wanted to leave Canterbury. For the
2017 State of Origin series, Klemmer played in all 3 matches off the interchange bench in the Blues 2–1 series loss. In the lead-up to Game 3, Klemmer was set to start at prop by coach
Laurie Daley and with
Andrew Fifita to come off the bench but rumours that Fifita blew up at Daley about the switch and he later reversed that decision and the Blues would later go on to lose the match 22–6. Klemmer was named as the winner of the
Brad Fittler Medal for being the Blues Best Player of the series. Klemmer was the second player to receive the award by playing all 3 matches of the interchange bench. Klemmer finished the
2017 NRL season with him playing in 21 matches and scoring 1 try for the Bulldogs. On 3 October 2017, Klemmer was selected in the 24-man Australia Kangaroos
2017 Rugby League World Cup squad. Klemmer played in all 6 matches of the tournament, including starting at prop in the World Cup Final match against
England in the hard-fought 6–0 victory at
Suncorp Stadium.
2018 In 2018, Klemmer was selected to play for New South Wales in the
2018 State of Origin series which The Blues emerged victorious winning 2–1. In game 3, Klemmer was placed on report for an attempted trip on Queensland player Ben Hunt. Klemmer was later fined $1350 but escaped suspension. Late in the year, Klemmer asked for a release from the final two years of his Canterbury contract. He was granted the release in November, signing a five-year contract with the
Newcastle Knights starting in 2019.
2019 On 7 October 2019, Klemmer was named in the Australian side for the upcoming
Oceania Cup fixtures.
2020 Klemmer played 21 games for Newcastle in the
2020 NRL season. He played in Newcastle's first finals game since 2013 which was a 46–20 loss against
South Sydney in the elimination final.
2021 Klemmer made 21 appearances for Newcastle in the
2021 NRL season as the club qualified for the finals. Klemmer played in Newcastle's elimination final loss to
Parramatta.
2022 In round 17 of the
2022 NRL season, Klemmer was sent off in Newcastle's 40–28 loss to
South Sydney. On 2 August, Klemmer was stood down by the Newcastle club for an on-field incident which happened during the clubs round 20 loss to
Canterbury. It was alleged that Klemmer refused to be substituted towards the end of the match and verbally abused a Newcastle trainer. On 16 November, Klemmer joined the
Wests Tigers on a three-year deal in a player swap with
Jackson Hastings.
2023 Klemmer played in all 24 of the Wests Tigers games for 2023. He was 9th in the competition for post-contact metres and 20th for run metres, leading the club in both statistics. The Wests Tigers would end up finishing with the
wooden spoon for a second straight year.
2024 In round 4 of the
2024 NRL season, Wests Tigers won 17–16 over Parramatta at the
Western Sydney Stadium. Parramatta's
Clinton Gutherson had a penalty conversion after the full-time siren to win the match for Parramatta however he missed the kick with camera footage showing Klemmer doing a
Bras d'honneur at Gutherson after the ball sailed wide of the posts. Klemmer was alleged by some fans online of cheating as he waved his arms and jumped in front of the goalposts as Gutherson moved to kick the ball, a distraction which is against the rules. Klemmer made 20 appearances for the Wests Tigers throughout the season as the club finished with the wooden spoon for a third consecutive season.
2025 On 13 March, Klemmer had signed a deal with the
St. George Illawarra Dragons for the rest of the 2025 season. Klemmer made his club debut for St. George Illawarra in round 4 of the
2025 NRL season against Melbourne. Klemmer played a total of 22 games for St. George Illawarra in the 2025 NRL season as the club finished 15th on the table. On 31 October it was announced that he had signed for
St Helens on a two-year deal.
2026 On 7 February, Klemmer made his club debut for St Helens in their 98–2 victory over
Workington Town in the third round of the Challenge Cup. == Statistics ==