[2] He made his senior debut as a second-half substitute in a 4–0 home victory over Inverness CT in the Scottish League Cup six days after his 16th birthday on 31 August 2022, becoming the youngest player in a major competition across the club's 136-year history.
[3][4] He was an unused substitute for the first team for several fixtures during the 2022–23 season, with his first appearance in the Scottish Premiership coming in a 3–0 loss to Rangers on 28 December 2022.
[10] With Paul McGinn and Stephen O'Donnell unavailable due to injury, 18-year-old Miller was named as team captain for Motherwell in the semi-final of the 2024–25 Scottish League Cup at Hampden Park (his first senior appearance at the national stadium);[11] they took the lead against Rangers, but ultimately lost 2–1.
[13][14] Miller made his debut for the Scotland under-16s aged 14 in August 2021 against England, in which he scored directly from the kick-off as play resumed at the start of the second half[15] – the strike was recorded on video, and was later awarded as the Goal of the Year in Scottish international football across all levels.
[19] Lennon is the son of former Scotland international footballer Lee Miller; his mother Donna died when he was five years old.