[2] Lennon entered the Professional Darts Corporation's Qualifying School in 2017, and picked up a Tour Card on the third day.

Lennon made it to the final of the 2018 Dutch Darts Masters, where he lost out to Michael van Gerwen.

In June 2019, Lennon alongside William O'Connor made it all the way to the final of the 2019 PDC World Cup of Darts where they eventually lost to Scotland 3–1.

[4] He enjoyed one of his finest days in the PDC on 22 February 2020 when he got to the semi-finals of Players Championship 5, only to lose narrowly to Gerwyn Price.

He missed his first World Championship in six years, but managed to stay within top 64 of PDC Order of Merit and continued on the Pro Tour in 2023.

Lennon came back from 0–2 on sets against Owen Bates in the First Round and won 3–2, giving him chance to keep his Tour card.