Sligo Rovers Mahon began playing for his local club
Sligo Rovers at underage under 17 level in 2015, where he played for 2 years before progressing to the under 19 side and then the first team in 2017. He made his first team debut on 17 April 2017 in a 1–0 win away to
Finn Harps in a
League of Ireland Cup tie at
Finn Park. That proved to be his only appearance of the season despite being named as an unused substitute in 19 league games. His
League of Ireland Premier Division debut came on 21 April 2018 in a 0–0 draw with
St Patrick's Athletic at
The Showgrounds. In June 2018 he earned a new two-and-a-half-year contract to keep him at the club until the end of the 2020 season. Mahon scored his first goal in senior football on 26 October 2018 in a 2–0 win over
Shamrock Rovers in the final game of the season. He made a total of 29 appearances during his breakthrough season, scoring 1 goal. On 2 February 2020, Mahon suffered a broken
tibia in a pre-season friendly against
Athlone Town. The injury kept him out of action for almost the entirety of the season but he returned to the side in October 2020 and made 5 appearances in all competitions by the season's end. On 29 October 2021, Mahon scored his first goal in just over three years when he opened the scoring in a 2–0 win over
Drogheda United. On 21 December 2021, he signed a contract with to keep him at the club until the end of 2023. Upon signing his new contract, Mahon stated that he had a desire to play abroad but for now his focus was on helping his club to win trophies. The following month however, the six figure release clause in his contract was met and he departed the club. He made 102 appearances in total for
Sligo Rovers during his time at the club, scoring 2 goals. He left the club only a year into his contract, with the club and player mutually agreeing to terminate his contract.
Return to Sligo Rovers On 13 January 2023, it was announced that Mahon had returned to
Sligo Rovers. In January 2024, Mahon suffered a ruptured
Achilles tendon which kept him out of action until the club's final 2 league games of the season. On 15 November 2024, he signed a new contract with the club until the end of 2026. He was made club captain ahead of the
2025 season. On 3 March 2025, he scored a 78th minute winner in a 2–1 victory over
Shamrock Rovers at
The Showgrounds, his first goal since 2021.
Waterford On 27 December 2025, it was announced that Mahon had signed for
League of Ireland Premier Division club
Waterford. ==Career statistics==