At the 2020 state election, MacMahon won the seat of South Brisbane from Labor's Jackie Trad, becoming the second Greens member in parliament alongside Michael Berkman.

[5] In 2024, MacMahon opposed the development of 14,000 high density homes near the Cross River Rail Station in Woolloongabba, 20% of which would be set aside for affordable housing.

[7] MacMahon argued that 20% was too low, that there was inadequate infrastructure in the area, the process was "extremely undemocratic" and that developers would profit.

"[7] In the 2024 Queensland state election, she lost her seat to Labor candidate Barbara O'Shea.

[2] On 12 February 2024, MacMahon was involved in a serious car crash in Kangaroo Point, suffering head and suspected spinal injuries.