On 18 May 2012, Fenlon signed his first professional contract with the Dons. His first involvement came as an unused substitute in a 3-1 League Cup First Round defeat at Stevenage on 14 August. On 21 August, he made his Football League debut as a half-time substitute for Warren Cummings in a 6–2 defeat to Burton Albion. He made his first start for the club four days later in a 5–1 defeat at Bradford City. Fenlon scored his first professional goal in a 3–2 defeat to Oxford United on 2 October. On 6 June 2014, it was announced that Fenlon had been offered a new contract but could not come to an agreement with the club. Manager Neal Ardley revealed the deal would have made him one of the club's highest earners.
Ross County
In July 2014, Fenlon signed for Scottish Premiership club Ross County. He made his debut on 10 in a 2–1 defeat to St Johnstone on 10 August. With County struggling in the league, Fenlon also featured in defeats to Partick Thistle, Dundee United and Aberdeen while being named as an unused substitute on two occasions. On 26 August, he recorded his first victory for the club in a 2-1 Scottish League Cup win at Stranraer. His final appearance for the club came in a 2-0 League Cup defeat to Hibernian on 23 September. On 28 November, Fenlon's contract was terminated by mutual consent.
In September 2016, Fenlon completed a move to Isthmian League Premier Division outfit Leatherhead. On 14 September, he made his debut in an Isthmian League Cup First Round victory over Walton Casuals. Fenlon made his league debut a week later in a 1–1 draw with Needham Market. Making 14 appearances in all competitions for the Tanners, his final outing came in a 3–1 victory at Hendon on 10 December.
Walton Casuals
On 30 December 2016, Fenlon joined Isthmian League Division One South side Walton Casuals. He made his debut the following day a 1–0 victory against Carshalton Athletic. Fenlon scored his first goal in almost five years, and his only for the club, in a 5–1 defeat at Faversham Town on 4 February 2017. He left the club at the end of the season.
Walton & Hersham
In July 2017, he joined local rivals Walton & Hersham in the Combined Counties Premier Division. Fenlon made his debut on the opening day of the season in a 7–0 FA Cup win against Mile Oak. On 10 October 2017, Fenlon became the club's first player to score at the Elmbridge Xcel Sports Hub – their new ground shared with Walton Casuals – in a 6–1 Combined Counties Challenge Cup victory against Badshot Lea. Five days later, he scored against from the penalty spot in a 2–0 win at Horley Town. On 21 October, Fenlon scored his third penalty of the season in a 3–3 draw against Balham.
