Having attended training sessions at Rangers, Miller signed for Celtic in 2010, aged 12. In addition to domestic tournaments, Miller featured in several international youth tournaments during his Celtic youth career such as the NextGen Series and the UEFA Youth League. In May 2016 he was named in a matchday squad for the first time and also signed a new contract running until 2019. Earlier that year he had been marked by the Scottish press as one of the 'three to watch' from the youth team. Miller made his league and competitive debut against Partick Thistle on 20 December 2016. He received praise for his performance in the match, which Celtic won. It was at this point that his manager Brendan Rodgers revealed that Miller, who had previously been considered a winger or forward in his youth career with the club, was retraining in a more defensive position. Rodgers also expressed his hope that Miller could flourish in his new role in a similar manner to his previous trainee Ryan Bertrand. He did not feature for the senior team again until 8 August 2017, when he played the full 90 minutes in a 5–0 win over Kilmarnock in the Scottish League Cup, with regular left-back Kieran Tierney filling in as a central defender. Miller started at left-back on 19 August 2017 in the Scottish Premiership, again against Kilmarnock, and was given man of the match by Sky Sports. His side won 2–0 with goals from James Forrest and Callum McGregor. He was selected in further home fixtures against St Johnstone in February 2018 and Kilmarnock again in May, being replaced in the second half by Tierney in both matches, which ended in 0–0 draws after Celtic made several other changes which appeared to affect their rhythm as a unit. Miller signed a new contract with Celtic in August 2018, tying him to the club until summer 2020 with the possibility of extension. Speaking on his future, he stated that his short-term aim was to secure a loan move to gain further experience.
Dundee loan
On 31 August 2018, Miller moved to fellow Premiership club Dundee on loan. He made his debut from the bench on 1 September in a 3–1 home defeat to Motherwell. On 23 October, he missed an opportunity to score his first senior goal when he sent a penalty kick wide of the post in an eventual 3–0 defeat to Heart of Midlothian. On 18 December he did score, netting Dundee's consolation goal in a 5–1 loss away to Aberdeen. His loan, intended originally to be for the full season, was curtailed at the end of December 2018.
Having returned to Celtic in summer 2019 Miller spoke of his hope of being in the plans of Neil Lennon, Celtic's new manager who had used Miller in friendly matches in his previous spell in charge of the club. However, when Kieran Tierney was sold in August 2019, Miller was overlooked at left back for a UEFA Champions League qualifier against CFR Cluj, the position instead being taken by midfielder Callum McGregor – who put in an uncertain performance as Celtic lost at home and were eliminated from the competition. Miller then turned down a move to Kilmarnock as part of a deal to bring Greg Taylor to Celtic, a signing which, along with improvement in the form of Boli Bolingoli, another new signing in the same position, further diminished the chances of Miller being selected. The slight prospect then disappeared entirely after Miller was injured in a reserve match on 17 September, requiring surgery the following week which would rule him out until the end of the season with no opportunity to impress the manager within the club or out on loan. His departure was confirmed in May 2020.
International career
Miller has been selected for Scotland at various youth levels. He made his debut for the under-21 team in March 2019.
Personal life
Miller was born in Glasgow and grew up in the city's Castlemilk district. He is the father of one daughter, Olivia, born in 2015.
