Norwich City fought off interest from Motherwell and Preston to sign Celtic defender John Kennedy on loan from 27th July 2008 until January 2009.

Kennedy came through Celtic's youth system and made his first-team debut during the 1999/00 season but it wasn't until the 2003/04 season that he really made his name.

He suffered a serious knee injury on his international debut for Scotland against Romania in March 2004 which kept him out of the game for three years. In late 2006 Kennedy resumed training and was named as a substitute for Celtic's Champions League match against A.C. Milan on 20 February 2007. He started his comeback against Kilmarnock on 22 April 2007, helping Celtic clinch the title. Four days later, he signed a new three year contract with Celtic. He injured his knee again in November 2007 against Shakhtar Donetsk in the Champions League which ruled him out for the rest of the season

John has 15 Scottish Under-21 caps

Kennedy returned to Celtic after further damaging his troubled knee in December 2008.On 13th November 2009, it was announced that John Kennedy was to retire from football, at the age of 26, on medical grounds. Kennedy said: “It is with great regret that I have made this decision. However, given the medical advice which I received, this is the right decision. Clearly, this is very disappointing for me and my family. However, this is something I will accept and come to terms with. I have enjoyed my time with Celtic immensely and I would like to thank everyone at the club and all the Celtic fans for all the support and backing they have given me during my time. It has been an absolute privilege to play for such a great club and of course I wish the team the very best for the future."

After retiring as a player, Kennedy joined the scouting network at Celtic, and has travelled over Europe to help find new players for the club

In the summer of 2010, Kennedy joined the board of Directors of the UK based charity Football Aid.

Kennedy gained coaching qualifications and worked as a coach with the club's U19 squad in season 2011-12, and the Development Squad (reserve team) from 2012 to 2014. He became first team coach in 2014 after the appointment of Ronny Deila as manager. He was appointed as Assistant Manager in February 2019 when Neil Lennon returned to the club as manager.

Celtic won the League title four times whilst he was first team coach.

Following Neil Lennon's resignation on 24th February 2021 with Celtic 18 points adrift of Rangers in the Premiership table, Kennedy was appointed Caretaker Manager

Following the departure of Celtic manager Ange Postecoglou to Tottenham Hotspur in June 2023, it was reported that Ange wanted Kennedy to join him at White Hart Lane and that Kennedy was looking for new challenges. However Celtic were blocking the move with potential new manager Brendan Rodgers keen to keep Kennedy as part of his backroom team (Football Scotland, 15 June 2023). In the end, Kennedy opted to stay at Celtic to work with Rodgers.