Miller progressed through the youth ranks at Isthmian League Premier Division club Carshalton Athletic and broke into the senior team for the 2013–14 season. On 15 April 2014, he made his debut in a 1–0 win at Leiston.[2] In December 2014, he was rewarded for his form and signed a part-time contract with the club.[3]
Miller appeared in every league match during the 2016–17 season.[4] On 25 March 2017, he scored his first hat-trick in a 4–1 win against Herne Bay.[5]
His appearances attracted the attention of several Football League clubs, and in May 2017, he became a part of Jamie Vardy's V9 Academy.[6] In July 2017, Miller trialled with Sheffield Wednesday.[7]
After being an unused substitute on 26 August 2017, his run of 49 consecutive league appearances came to an end.[8] However, he marked his return to action with a brace against Corinthian-Casuals, an FA Cup hat-trick against Pagham and the opening goal in 2–1 win against Faversham Town.[9][10][11]
On 20 November 2017, he scored his second hat-trick in a 6–2 win against Egham Town in the Surrey Senior Cup.[12] He also netted all three goals in a 3–0 win against Sittingbourne on 6 January 2018.[13]
Hamilton Academical
On 20 January 2018, Carshalton Athletic announced that Miller had agreed terms with Scottish Premiership side Hamilton Academical.[14] Nine days later, Hamilton confirmed the signing for an undisclosed fee.[15] After being named as an unused substitute on three occasions, Miller made his professional debut in a 2–0 win against Motherwell on 10 March 2018.[16]
Miller had a successful 2018–19 season with Hamilton Academical where he finished as the club's top goalscorer.
In February 2019 Hamilton Academical activated their option to extend his contract to keep him until summer 2020.[17]
Rotherham United
Miller joined Rotherham United on 5 July 2020 on a two-year deal.[18] On 22 August 2020, Mickel made his first start for Rotherham in a pre-season fixture scoring a header in a 3-0 win v Mansfield.
Miller joined Northampton Town on loan until the end of the 2020–21 season on 14 January 2021.[19][20]
Injuries hampered his 2022–23 season, only making 12 appearances in Argyle's league title-winning campaign. He had to wait until 19 November to make his debut, as a substitute in Argyle's 2–2 draw away to Burton Albion.[23] On 2 March 2023, he featured in the EFL Trophy Final for a consecutive season, this time coming out on the losing side as Argyle lost 4–0 to Bolton Wanderers.[24]
The 2023–24 season in the EFL Championship saw Miller feature more regularly, making more appearances by Christmas than he'd made in the entirety of the previous season. His versatility saw him deployed as both a wing-back in a 3–4–3, and as an out-and-out left-back in a 4–3–3.[25] He was offered a new contract at the end of the season.[26]
On 1 July 2024, Plymouth Argyle announced that Miller's contract had expired and that he would be departing the club.[27]
Huddersfield Town
On 1 July 2024, Miller signed for recently relegated League One side Huddersfield Town on a two-year contract.[28]