Johnson is regarded as one of New Zealand’s greatest rugby league players, often compared to Benji Marshall and Stacey Jones.

[3] From the Whangaparāoa Peninsula, Johnson attended Whangaparaoa Primary School,[4] as well as Orewa College, and played many sports, including basketball, rugby union and Australian rules football (representing New Zealand at Under 16 level),[5] but made his name playing touch and Rugby League.

[6] Johnson played for the Hibiscus Coast Raiders in the Auckland Rugby League competition before being signed by the New Zealand Warriors in 2009.

[11] In his next match in Round 14 against the Wests Tigers at Mt Smart Stadium, Johnson scored his first NRL try in the Warriors 26–22 loss.

[14] The Warriors lost to the Sea Eagles 24–10 in the 2011 NRL Grand Final however Johnson played a tremendous game and almost sparked a surprise comeback with a role in both tries and a few outstanding runs.

[17] Due to Kieran Foran's hamstring injury, Johnson was selected as halfback for the New Zealand national team in the 2012 Anzac Test.

[21] For the 2013 Anzac Test, Johnson was selected to play halfback for New Zealand, kicking two goals from two attempts in their 32–12 loss against Australia at Canberra Stadium.

[22] In Round 14 against the Sydney Roosters, Johnson chased down Michael Jennings after the latter picked up a loose ball and sprinted toward an undefended try line, Warriors later won the match 23–12 at SFS.

[25] Following the Kiwis 34–2 World Cup Final loss to Australia at Old Trafford,[citation needed] Johnson finished the tournament as the top points-scorer with 76 points (4 tries and 30 goals).

[34] Johnson was selected for New Zealand in the 2015 Anzac Test against Australia at halfback, scoring a try and kicking 3 goals in the Kiwis 26–12 win at Suncorp Stadium.

[36] In Round 20 against the Manly Sea Eagles, Johnson suffered a season-ending ankle injury while scoring a try in the Warriors 32–12 loss at Mt Smart Stadium.

[39] Shaun Johnson was part of the Four Nations 2016 New Zealand Kiwis side that made it to the Final of the tournament, losing to the Australian Kangaroos 34–8.

Shaun Johnson became New Zealand rugby league's greatest test points scorer as the Kiwis beat Scotland 74–6.

In round 15, Johnson scored a try but missed all three attempts at goal as Cronulla suffered a shock 14–12 loss against last placed Canterbury-Bankstown at ANZ Stadium.

[42][43][44] Johnson made a total of 18 appearances for Cronulla in his first season at the club as they finished in 7th spot on the table and qualified for the finals.

[45] In round 19 of the 2020 NRL season, Johnson was taken from the field during the second half in Cronulla's match against the Sydney Roosters with a serious leg injury.

[46] In round 7 of the 2021 NRL season, Johnson made his return to the Cronulla side in an 18–12 loss against Canterbury at Kogarah Oval.

[52] In round 5 of the 2022 NRL season, Johnson kicked the winning field goal for the New Zealand Warriors in their 25–24 golden point victory over North Queensland.

[53] In round 8, Johnson kicked a field goal in golden point extra time to win the game for New Zealand 21–20 over Canberra.

In round 5 of the 2023 NRL season, Johnson kicked a penalty goal with less than one minute remaining as the New Zealand Warriors defeated Cronulla 32–30 at Shark Park.

In round 21, Johnson kicked a field goal in golden point extra-time to win the game for the New Zealand Warriors 21–20 against Canberra.

[68] On October 10, 2024, he made his return for international duty since 2019 to replace Jahrome Hughes for the New Zealand Kiwis’ Pacific Championships campaign against the Kangaroos and Tonga.