McMillan was part of the Irish Colleges team that travelled to Thailand in the summer of 2007, where he finished top scorer for his country with two goals.
On 30 October 2009, UCD successfully completed their return to the League of Ireland Premier Division with a 5–0 victory away to Athlone Town.
McMillan played a huge role in this extraordinary season, making more appearances than any other player in the squad, while also having the best defensive record in the league, conceding only 21 goals in 33 games and keeping 16 clean sheets.
McMillan scored the winning penalty against South Dublin rivals Shamrock Rovers in the League cup on 25 April 2011.
McMillan signed for Bohemians on 12 January 2012[3] and made his competitive debut in a 2–1 win over Portadown in the Setanta Sports Cup.
McMillan opened his goal account for Bohemians on 15 July in a league match vs former club UC Dublin.
[6][7] He made his debut on 4 March 2013 in the Setanta Cup quarter final 5–0 victory over Glentoran, with McMillan scoring the fifth with a fine header.