He joined Oldham's Centre of Excellence as a first year scholar in May 2009,[3] having previously been at Linfield from the age of ten, progressing to reserve team football at the age of fifteen and then making a first team appearance at the age of sixteen.[4]
He made his Football League debut on 13 March 2010 as a substitute during a 2–2 draw with Wycombe Wanderers[2] and started his first match for the club on 30 March in a 3–2 defeat against Brentford.[5] He was offered a two-year professional contract by Oldham in April 2010, twelve months ahead of his scholarship expiring.[6]
Millar's chances of establishing a first team place for Oldham Athletic in the 2011–12 season were dealt a blow as he was sent off in a reserves team match against Morecambe for clashing with the keeper Lawrie Walker just after recently being told by the manager Paul Dickov that he was going to be given a chance to show his ability at the end of the 2010–11 season.[7]
Millar had scarcely featured for Latics throughout the 2012–13 season but found a place in the team when Paul Dickov left the club and new man Lee Johnson arrived. Millar played a crucial part in Oldham's survival, assisting several goals, most notably Matt Smith's late winner against Bury FC at Gigg Lane. Millar scored his first & only goal for Oldham during the last game of the season against Leyton Orient, slotting the ball into the bottom right hand corner to make it 1–1.[8]
At the end of the 2013–14 season, a season in which he was limited to a few substitute appearances, Millar was released after 6 years at the club.[9]
Chorley (Loan)
In October 2012 he joined Chorley on loan for a one-month period[10] and made his club debut on 16 October in a Northern Premier League match against AFC Fylde.[11]
In August 2014, Linfield manager Warren Feeney announced the signing of Millar on a free transfer. Millar signed a 2-year contract following a trial period with the club.[13]
In June 2011 he received his first call-up to the Northern Ireland Under-20 team, along with teammate Carl Winchester for July's Milk Cup tournament.[17]