Celtic's interest was confirmed by chief executive Peter Lawwell in January 2008 who explained that the club planned on signing Mizuno, if he was granted a work permit.
After initially being denied a permit, Gordon Strachan took part in an appeal hearing in which he described his other Japanese import Shunsuke Nakamura as a "flawless performer" and "The best player I have ever worked with, and that includes [Kenny] Dalglish and [Gary] McAllister", and claimed that Mizuno was a young player of immense talent and potential. The appeal was successful and Celtic completed the signing of Koki Mizuno on 29 January 2008. On 18 July 2008, Mizuno made his debut in a 2–0 pre-seasonfriendly win over Southampton. On 8 November, he made his league debut for Celtic in a 2–0 victory over Motherwell, replacing Scott McDonald in the 88th minute and, on 21 December 2008, he made his first start in a game against Falkirk He scored his first goal for Celtic in the 90th minute of the match, assisted by a pass from Shunsuke Nakamura. On 1 July 2010, Mizuno was released by Celtic.
In the summer of 2010, Mizuno signed for J. League Division 2 side Kashiwa Reysol. In his first game for Kashiwa, he suffered a knee ligament injury which sidelined him for the rest of the 2010 season.