He played for the Indigenous All Stars and the Prime Minister's XIII at representative level, and won the 2014 NRL Grand Final with the South Sydney club.

Johnston was born in Sydney, New South Wales, Australia, and has Aboriginal and Lumi ancestry, Sandaun Province, Papua New Guinea[3] and Saibai Island in the Torres Strait.

[4] He played his junior rugby league for La Perouse United and attended Endeavour Sports High School (where he completed the HSC) before being signed by the South Sydney Rabbitohs.

[5] In round 8 of the 2014 NRL season, Johnston was selected to make his NRL debut for the South Sydney Rabbitohs over demoted Souths winger Nathan Merritt against the Brisbane Broncos on the wing, scoring a try in the 28–26 win at Suncorp Stadium on Anzac Day.

[12] 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.

[23] 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 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.

In round 21, Alex Johnston was switched back to fullback after poor performances by Souths who were on a three-game losing streak.

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.

[29][30] In round 12 of the 2020 NRL season, Johnston scored a hat-trick as South Sydney defeated St. George 32–24 at Kogarah Oval.

It ended months of speculation about the players future as he had been linked to Melbourne, Wests Tigers and North Queensland.

[34] In the final round of the 2020 NRL season, Johnston scored five tries as Souths defeated bitter rivals the Sydney Roosters 60–8 at ANZ Stadium.

[36] Johnston made a total of 22 appearances throughout the season as Souths reached their third straight preliminary final but lost 20–16 against Penrith.

[41] 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.

[55] 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.

[57] 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.

[60] Following South Sydney's round 3 victory over Canterbury in the 2024 NRL season, it was announced that Johnston would be ruled out indefinitely with a hamstring injury.