Miller has previously played for Celtic, Dundee, Ayr United, Harrogate Town, Notts County, Chesterfield and Greenock Morton.

[10] 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.

[11] 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.

[14] He was selected in further home fixtures against St Johnstone in February 2018[15] 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.

[20] 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.

[23] On 31 January 2019, Miller joined Scottish Championship club Ayr United on loan until the end of the 2018–19 season.

[25] 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.