Early years Between 2000 and 2007, McMillan attended
Wesley College, Dublin where he studied for his
Leaving Cert. As a schoolboy he played for Templeogue United and Mount Merrion Youths. He initially attended
UCD games when Templeogue United supplied the club with ballboys. Mount Merrion Youths were effectively UCD's schoolboy team and his brother
Evan also played for them.
UCD Between 2007 and 2010, McMillan attended
University College Dublin on a sports scholarship and obtained a degree in architecture. He also played for
University College Dublin A.F.C. at both intervarsity and national level. In 2006 McMillan and his brother
Evan began playing for UCD in their
Leinster Senior League team. McMillan subsequently progressed through the ranks into the senior team. He made his
Premier Division debut during the
2008 season, coming on as a substitute in an away game against
Cork City on 26 September. During his time at UCD he helped the club win three
Collingwood Cups, two CUFL Premier Division titles, two
A Championships and the
2009 League of Ireland First Division title. In the
2008 A Championship final, McMillan and his brother scored the two goals in an extra time win against
Bohemians A. McMillan also scored 12 league goals during UCD's successful 2009 First Division season. In
May 2010 McMillan was named
League of Ireland Premier Division Player of the Month after he enjoyed a prolific run of form. McMillan scored against
Dundalk,
St Patrick's Athletic and
Galway United. He also scored twice in a 6–0 win over
Bray Wanderers.
St Patrick's Athletic During the
2011 season McMillan played for
St Patrick's Athletic where he once again linked up with his brother
Evan. He made 13 league starts but also gained a reputation as a super-sub, adding 14 more appearances from the bench. He also played for St Pat's during their
2011–12 UEFA Europa League campaign, scoring in the
second qualifying round against
Shakhter Karagandy. On 10 October 2011, McMillan helped St Pat's win the
2011 Leinster Senior Cup, scoring in the
final against
Bohemians at
Dalymount Park.
Olympic In 2012 while studying for his master's degree in architecture, McMillan spent an
Erasmus semester at the
University of Queensland.
Sligo Rovers In July 2013 McMillan signed for
Sligo Rovers. McMillan scored on his debut for Rovers, a 3–1 defeat to
Cork City. This proved to be his only goal in 13 league appearances over the remainder of the
2013 season. He also played for Rovers in their
2013–14 UEFA Champions League games against
Molde FK. However having played for
UCD in the early rounds, McMillan was cup-tied and he missed out on Rovers successful
2013 FAI Cup campaign.
Dundalk In December 2013 McMillan signed for
Dundalk. During the
2014 season he scored 15 goals and helped Dundalk win a
League of Ireland double as the club won both the Premier Division title and the
League of Ireland Cup. He also played for Dundalk in their
2014–15 UEFA Europa League campaign. In the
2015 season, McMillan scored 14 times as he helped Dundalk win a second double. This time Dundalk retained the Premier Division title and won the
2015 FAI Cup. In Dundalk's
2015–16 UEFA Champions League campaign, McMillan scored their only goal as they lost 2–1 on aggregate to
BATE Borisov in the second qualifying round. During Dundalk's
2016–17 UEFA Champions League campaign, McMillan scored three times against
Fimleikafélag Hafnarfjarðar in the second qualifying round. In the third qualifying round he scored twice against BATE Borisov. McMillan goals secured a 3–1 aggregate win for Dundalk and guaranteed the club a place in the
2016–17 UEFA Europa League group stage. McMillan also became the
League of Ireland's all-time leading scorer in the
UEFA Champions League, overtaking
Jason Byrne and
Glen Crowe. After scoring two hat-tricks against
Derry City and
Longford Town in the
2016 Premier Division and scoring five goals in the UEFA Champions League, McMillan was named Premier Division Player of the Month for
July 2016. McMillan was joint top-scorer in the
2016–17 UEFA Champions League qualifying phase and play-off round with five goals. While playing for Dundalk, McMillan has also been employed as an architect by O'Brien Finucane Architects, a company based in
Dawson Street,
Dublin. He made his debut on 27 January 2018, as a substitute at home to
Partick Thistle, however he only lasted 21 minutes before having to go off injured. The hamstring injury he sustained kept him out for nearly three months. On 5 May 2018, McMillan scored his first goal for St Johnstone, in a 5–1 win away at
Motherwell. He moved on loan to
Hamilton Academical in January 2019. On 4 September 2019, McMillan joined
Falkirk on a season-long loan. McMillan left the Saints in July 2020.
Return to Dundalk After two years in Scotland, McMillan signed again with
Dundalk on 29 July 2020. He scored the winning goal against
Inter Club d'Escaldes in the
2020 Europa 2nd qualifying round. Dundalk went on to qualify for the
2020–21 UEFA Europa League group stage and he scored two goals from the penalty spot in Dundalks 4-3 lost against
Rapid Vienna at the
Allianz Stadion. This equalled
Glen Crowe's European record of 11 goals. On 6 December 2020, McMillan scored a hat-trick in the
2020 FAI Cup Final in a 4–2 win over
Shamrock Rovers at the
Aviva Stadium, becoming the third player ever to achieve the feat. He was part of the starting 11 as his side won the
2021 President's Cup, beating
Shamrock Rovers on penalties after a 1–1 draw at
Tallaght Stadium on 12 March 2021. He scored in both legs to help defeat
Newtown A.F.C. and
FCI Levadia Tallinn in the
2021–22 UEFA Europa Conference League qualifying phase and play-off round.
Retirement On 23 February 2023, McMillan publicly announced his retirement from professional football at the age of 34, stating "
I would never have dreamed of coming through and doing the things I did" during his career. ==International career==