2014 In round 8 of the
2014 NRL season, Johnston was selected to make his
NRL debut for the
South Sydney Rabbitohs on the against the
Brisbane Broncos, at
Suncorp Stadium on Anzac Day replacing
Nathan Merritt and scoring a try in his team's 28–26 win. He continued to perform on both the left and right wing throughout the year. At times, Johnston was referred to as the successor of
Greg Inglis, for his ability to play fullback as well as wing. In round 21, Johnston scored his first ever hat-trick against the
Newcastle Knights in the Rabbitohs 50–10 victory. On 5 October 2014, in Souths
2014 NRL Grand Final against the
Canterbury-Bankstown Bulldogs, Johnston played on the wing and scored the first try of the match to lead the South Sydney Rabbitohs 30–6 victory. Johnston finished his brilliant debut year in the
NRL as the competitions highest tryscorer with 21 tries in 18 matches. In September 2014, he received an offer to play for his maternal grandmother's native . On 14 October 2014, Johnston was selected for the
Australia Kangaroos 2014 Four Nations squad but didn't make an appearance in any of Australia's four matches.
2015 On 13 February 2015, Johnston played on the wing for the
Indigenous All Stars against the
NRL All Stars in the
2015 All Stars match at
Cbus Super Stadium, the Indigenous side winning 20–6. On 23 February, he played for the Rabbitohs in the
2015 World Club Challenge match against
2014 Super League Grand Final premiers
St. Helens, playing on the wing in the Rabbitohs' 39–0 win at
Langtree Park. For the
2015 Anzac Test, he was selected to make his debut for
Australia against
New Zealand, playing on the wing in the Kangaroos' 12–26 loss at
Suncorp Stadium. On 13 May, he re-signed with South Sydney on a 2-year contract to the end of the 2017 season. On 8 July, he was selected to be 18th man for the
New South Wales team for Game 3 of the
2015 State of Origin series. He finished off the
2015 season as South Sydney's highest try-scorer with 17 tries in 25 matches. On 26 September, he played for the
Prime Minister's XIII against
Papua New Guinea, playing on the wing and scoring a try in his team's 40–12 win at
Port Moresby. On 15 December, he was named on the wing for the Indigenous All Stars team to play the
World All Stars on 13 February 2016.
2016 On 13 January, Johnston was named in the emerging
New South Wales Blues squad. In February, he played for South Sydney in the
2016 NRL Auckland Nines, scoring the opening try of the competition. He was selected on the wing and scored four tries in South Sydney's first three matches of the season before sustaining a
hamstring injury that saw him sit out for several weeks. In his return game in round 10 against the
Parramatta Eels, he played in his preferred position at fullback in a 22–20 win. He was renamed in the fullback position in round 15, once again against the Eels. He was then moved back and forth as the club attempted to find a line-up that would win them. In round 23, Johnston played on the wing when South Sydney broke their nine-game losing streak with a 41–22 win over the
New Zealand Warriors. He played their for the next three final games which were all won by South Sydney. Johnston finished the 2016 season with 11 tries from 18 matches.
2017 In round 1 of the 2017 season, South Sydney fullback, Greg Inglis suffered what was deemed to be a season-ending injury to his
anterior cruciate ligament (ACL) in the club's 18–34 defeat by the
Wests Tigers. In round 2, Johnston was named at fullback for the club's clash with the
Manly-Warringah Sea Eagles. Johnston scored two tries in the position in his sides' 18–38 win at
Brookvale Oval. On 14 June he re-signed with the South Sydney club until the end of the 2020 season. On 2 July he scored five tries against
Penrith to be the first Souths player since Nathan Merritt in 2011 to achieve five tries in a match. In round 21, Alex Johnston was switched back to fullback after poor performances by Souths who were on a three-game losing streak. He scored an amazing try in South Sydney's 32 - 18 loss against Canberra. He then went on to score six tries in his next three games and was the leading try scorer with 2 rounds to go. However, against the New Zealand Warriors in round 24, Johnston injured his hamstring as he was scoring a try in the 36 - 18 win. He was ruled out for the rest of the season, with South Sydney losing their last two matches. Johnston finished season 2017 with him playing 22 matches and scoring 22 tries, the second highest tryscorer that year. Johnston expressed an interest in representing Papua New Guinea at the
2017 Rugby League World Cup, but was unavailable due to injury.
2018 Johnston was part of the South Sydney side which returned to form in 2018 finishing 3rd on the table at the end of the regular season. Johnston made 24 appearances for Souths scoring 8 tries as the club finished one game short of the grand final losing to Eastern Suburbs 12–4.
2019 Johnston began the
2019 NRL season as South Sydney's first choice fullback. South Sydney started the year off winning 8 of their first 9 games. Following Souths 19–18 loss against Penrith in round 14, Johnston was ruled out with a knee injury. In round 24, Johnston returned to the Souths side and scored a try as they defeated the
New Zealand Warriors 31–10 at
Mt. Smart Stadium. At the end of the 2019 regular season, Souths finished third on the table and qualified for the finals. Johnston scored a try in the club's 34–26 victory over Manly-Warringah in the semi-final at
ANZ Stadium. The following week, Johnston played in South Sydney's preliminary final loss against the
Canberra Raiders at
Canberra Stadium.
2020 In round 12 of the
2020 NRL season, Johnston scored a
hat-trick as South Sydney defeated St. George 32–24 at
Kogarah Oval. In round 14, Johnston scored two tries as South Sydney defeated North Queensland 31–30 at the
Queensland Country Bank Stadium. The following week, Johnston scored two tries as Souths defeated
Manly-Warringah 56–16 at
ANZ Stadium. In mid-2020, it was reported that Johnston was likely to leave South Sydney at the end of the season. It is believed that several clubs were interested in signing Johnston, particularly
Melbourne and
Wests Tigers. Johnston wrote a letter to South Sydney's coach
Wayne Bennett expressing how much the club meant to him, which convinced Bennett to re-sign Johnston. In the 2020 elimination final, Johnston scored two tries for Souths in their 46–20 victory over
Newcastle at
ANZ Stadium. Johnston made a total of 22 appearances throughout the season as Souths reached their third straight preliminary final but lost 20–16 against Penrith. Johnston finished as the year's top try scorer with 23 tries.
2021 In round 1 of the
2021 NRL season, he scored two tries in a 26–18 loss against Melbourne. In round 12, Johnston scored a
hat-trick in South Sydney's 38–20 victory over
Parramatta. In round 14, Johnston scored his second
hat-trick of the season in a 24–10 victory over
Newcastle. The following week, he scored two tries for South Sydney in a 46–0 victory over Brisbane. In round 16, he scored two tries in a 38–22 victory over
Wests Tigers at an empty
Leichhardt Oval, which took Johnston to the top of the try scorers list. In round 17, Johnston scored a
hat-trick during South Sydney's 46–18 victory over
North Queensland. In round 18, he scored two tries for South Sydney in a 32–24 victory over Canterbury. On 19 July, Johnston was ruled out from playing for a month with a hamstring injury. In round 24, Johnson scored a
hat-trick during South Sydney's 54–12 victory over arch-rivals the
Sydney Roosters. In the 2021 Finals Series, Johnston scored two tries for South Sydney in their preliminary final victory over Manly which booked Souths a place in the grand final for only the second time since their 1971 triumph. On 27 September, Johnston was awarded the Ken Irvine Medal for being the league's top try scorer with 27 tries. Johnston scored his 30th try for the season (including finals) in South Sydney's
2021 NRL Grand Final loss to Penrith. In doing so, he broke the record for most tries scored by a Souths player in a season which was held by
Les Brennan for over 60 years. On 18 October, Johnston signed a contract extension with the South Sydney club until at least the end of the 2025 season.
2022 In round 6 of the
2022 NRL season, he scored two tries in South Sydney's 36–16 victory over
Canterbury. In round 12 of the
2022 NRL season, he scored a
hat-trick in South Sydney's 44–18 victory over
West Tigers to become South Sydney's all-time leading try scorer. In round 14, Johnston scored a
hat-trick in South Sydney's victory over the
Gold Coast. In round 16, Johnston scored two tries in a 30–12 victory over
Parramatta. In round 19, Johnston scored two tries for South Sydney in their 24–12 victory over
Melbourne. In round 20, Johnston scored one try in a 21–20 loss to
Cronulla to enter the top 10 try scorers in NRL history. Johnston played a total of 25 matches for South Sydney across the season but he missed the clubs preliminary final defeat to eventual premiers
Penrith through injury. Johnston finished as the competitions top try scorer with 30 tries, and the first player in history to score 30 tries in a season twice.
2023 In round 10 of the
2023 NRL season, Johnston played his 200th first grade game and scored two tries in South Sydney's 28–12 victory over Melbourne. In round 14, Johnston scored a
hat-trick in South Sydney's 46–28 victory over the Gold Coast. In round 22, Johnston scored another
hat-trick in South Sydney's 32–18 victory over the Wests Tigers in Tamworth.
2024 Following South Sydney's round 4 victory over Canterbury in the
2024 NRL season, it was announced that Johnston would be ruled out indefinitely with a hamstring injury. In round 11, Johnston made his return to the team and scored a try in their 28-22 loss to North Queensland. On 25th May 2024 against Parramatta, Johnston scored his 190th try, which put him equal second with
Billy Slater on the all time scorers' leaderboard. In round 15, Johnston scored his 191st try which was the most by any player at a single club in the competition's 116-year history. In round 21, Johnston was taken from the field with a season-ending Achilles injury.
2025 In round 4 of the
2025 NRL season, Johnston made his long awaited comeback in South Sydney's 28-18 upset victory over Penrith scoring a try. Johnston was later taken from the field during the first half with an injury and did not return for the rest of the match. Johnston scored three tries in a narrow loss to the New Zealand Warriors in round 13 of the 2025 NRL season. In scoring three tries, Johnston become the second player in NSWRL/NRL history to score 200 career tries. In round 16, Johnston scored four tries for South Sydney in their 25-24 golden point extra-time loss against Melbourne. On 22 July, the Rabbitohs announced that Johnston had signed a two year extension with the club.
2026 During round 2 of the NRL season, Johnston broke the all time record of try scoring surpassing
Ken Irvine by scoring his 213th try of his NRL career. Fans stormed the field to celebrate with Johnston in the moment. The sitting
Prime Minister of Australia Anthony Albanese, a life-long fan of the South Sydney club, was among those who entered the playing field. In round 8 against the Storm, Johnston scored 2 tries in his 250th NRL game. The match coincided with South Sydney's first ever victory against the Storm in Melbourne. On 28 April, it was reported that Johnston had signed with the PNG Chiefs from 2028. In round 9 against the Knights, Johnston crossed for another double in a thrilling 42-38 loss. The double brought his streak to at 8 consecutive games with at least one try. Matching his personal best set back in 2022. On 6 May, the
Papua New Guinea Chiefs officially announced the signing of Johnston on a one-year deal from 2028, he was the clubs second signing. ==Try-scoring record==