[2] Lennon made his first team debut on 8 August 2017, starting a Leinster Senior Cup game against Firhouse Clover, scoring after 15 minutes when he smashed a shot into the top corner from 30 yards to help his side to a 2–1 win.

[5] Lennon made his League of Ireland Premier Division debut on 26 February 2018 in a 2–0 loss to Waterford, replacing Owen Garvan after 50 minutes.

[9] For his performances, Lennon was voted as St Patrick's Athletic Young Player of the Year by the club's supporters.

[13] Lennon started in the 2022 President of Ireland's Cup against Shamrock Rovers at Tallaght Stadium on 11 February 2022, as his side lost 5–4 on penalties after a 1–1 draw.

[21] On 3 June 2024, Lennon scored his side's second goal in a 3–2 loss to Dundalk, blasting home a volley from a Cian Kavanagh knockdown from a corner.

[33][34] On 21 May 2019, Lennon was called up to the Ireland Under-21 squad for the prestigious 2019 Toulon Tournament in France where they were drawn against China, Mexico and Bahrain.