In June 2019, Kelly joined West Coast as an international rookie, together with her sister Grace.[2][3][4][5][6] Making her debut in round 1, 2020 against Collingwood, Kelly was a part of West Coast's inaugural AFL Women's team.
In June 2022, both Kelly sisters left the Eagles; Niamh was traded to Adelaide, while Grace departed for St Kilda.[7]
Kelly had a career-best year in 2023. After winning the second-ever Showdown Medal awarded in the AFLW in round one of the season, Kelly recorded a personal-best 28 disposals and 99 AFL Fantasy points in round 8 against Brisbane. The following week, Kelly scored a match-winning goal against an in-form North Melbourne to secure her team a top-two finish and a home qualifying final.[8]
Kelly was rewarded for her outstanding form in 2023 with selection in the All Australian squad.