Monaghan made her county debut in 2016, for Hampshire against Somerset in a Twenty20 Cup match.[3] In her first County Championship match that same season, she hit what remains her List A high score, of 43*.[4] She was part of the Hampshire side that gained promotion from Division 2 of the County Championship in 2017, and subsequently won Division 1 the following season in 2018.[5] In 2017 she also took her best List A bowling figures, of 4/22 against Staffordshire.[6] She played six matches for the side in the 2021 Women's Twenty20 Cup, scoring 42 runs and taking two wickets.[7][8] She was ever-present in the 2022 Women's Twenty20 Cup, scoring 76 runs and taking four wickets.[9][10] She joined Oxfordshire for the South Central Counties Cup, a 50-over tournament involving the counties in the South of England.[11] She top-scored for Oxfordshire with 44 from 44 deliveries in the opening round of the tournament.[12] She played six matches for Hampshire in the 2023 Women's Twenty20 Cup, taking four wickets.[13]
In 2020, Monaghan played for Southern Vipers in the Rachael Heyhoe Flint Trophy. She appeared in all 7 matches as her side won the competition, scoring 11 runs and bowling 4 overs.[15][16] She played seven times for the side in 2021, across the Rachael Heyhoe Flint Trophy and the Charlotte Edwards Cup, scoring 35 runs and taking one wicket.[17][18] She also played for London Spirit in The Hundred, appearing in six matches.[19] Monaghan missed her university graduation to be at London Spirit's match against Northern Superchargers at Lord's, so the team instead celebrated together on the dressing room balcony after the match.[20] She played three matches for Southern Vipers in 2022, all in the Rachael Heyhoe Flint Trophy.[21] She also played six matches for London Spirit in The Hundred.[22] In April 2023, it was announced that Monaghan had signed her first professional contract with Southern Vipers.[23]