"I had been with Cardiff City since the age of nine - my entire career. But my contract ran out and I think it was time to move on.
"I have some fantastic memories of Cardiff. It's a great club, and my heartfelt wish is that they can achieve Premier League status.
"I just think the time was right to move on and have a look at my career."
Ledley is now in Amercia on Celtic's pre season tour. They face Philladelphia, Man Utd, Seattle and Sporting Lisbon before coming home for a Champions League qualifier. Then they face a double header at The Emirates against Olympic Lyonnais and Arsenal before the SPL starts on August 14th.