A left-arm orthodox bowler, Jonassen has been a member of the national women's team since 2012, going on to win four ICC T20 World Cups while becoming the fourth woman to have taken 100 One Day International wickets for Australia.

Jonassen was born in Emerald, a rural town in the Central Highlands Region of Queensland, but grew up approximately 270 km (170 mi) away in the coastal city of Rockhampton.

Making her WNCL debut against the New South Wales Breakers just three weeks after her 16th birthday (scoring 12 not out off 16 balls and taking 0/21 from three overs in an eight-wicket loss),[6] she went on to play every game of the season and finished with twelve wickets at an average of 19.83.

[8][9] She won the same award again, as well as the league-wide Player of the Year title, for the 2014–15 season in which she took the second-most wickets (eleven at an average of 13.36) and scored 197 runs at 49.25 despite missing several games due to injury.

[14] Jonassen signed with her local Women's Big Bash League (WBBL) team, the Brisbane Heat, as a marquee player for the competition's inaugural season.

[17] Two seasons later, Jonassen was at the centre of an iconic moment during the first WBBL|04 semi-final at Drummoyne Oval against the Sydney Thunder, bowling the final delivery of the match to Nicola Carey who initially appeared to hit the ball for a game-winning six until an unlikely catch by Haidee Birkett was taken metres inside the boundary rope.

[19] Jonassen enjoyed an outstanding WBBL|05 campaign, notably increasing her output with the bat (scoring 419 runs at an average of 38.09)[20] for which she credited an "honest conversation" with new coach Ashley Noffke.

[22][23] In a four-wicket semi-final win over the Melbourne Renegades at Allan Border Field, Jonassen earned Player of the Match honours by taking 1/25 and scoring 38 off 23 balls.

[27] On 25 January 2012, Jonassen made her One Day International debut at the Sydney Cricket Ground against New Zealand in a match that was abandoned during the 23rd over due to rain, bowling just nine deliveries and finishing with figures of 0/5.

[31] Jonassen was named in Australia's squad for the 2013 Women's Cricket World Cup but, after failing to recover quickly enough from knee surgery on her medial meniscus in early January, she was withdrawn days before the team traveled to India for the tournament.

[43] Resuming the second day on a score of 95, she could only manage four more runs before being dismissed lbw by Katherine Brunt on 99 off 197 balls and thus falling agonisingly short of a debut Test century.

[53] In a blog post for The Athletes Voice, she described the experience of having to "find ways to contribute to the team without being on the field" as a mental health battle which led to "some pretty dark places for a while".

[60] During a 110-run victory against Sri Lanka at Allan Border Field on 7 October 2019, Jonassen became the fourth woman to take 100 ODI wickets for Australia,[61] joining Cathryn Fitzpatrick, Lisa Sthalekar and Ellyse Perry.

[63] Jonassen managed her first T20I five-wicket haul in the final of the 2020 Tri-Nation Series at the Junction Oval, taking 5/12 off four overs to "completely swing the momentum of the game"[64] and help her team defeat India by eleven runs.

[65][66] In the opening fixture of the 2020 ICC Women's T20 World Cup at Sydney Showground Stadium, also against India, she finished with bowling figures of 2/24 but Australia would lose the match by 17 runs.