Mr Cook, who cited personal reasons for his departure, will remain a member of the club's advisory committee and continue to serve as a trustee of the Birmingham City Foundation.
In a statement on the website, Birmingham City said: “The Club extends its deepest gratitude to Garry for his leadership and numerous contributions, including enhancing the Club’s infrastructure and fostering positive experiences for fans and colleagues.”
Chairman Tom Wagner said: “We thank Garry for everything that he has done during his time as chief executive officer. His passionate leadership and commitment to the club that he has supported from boyhood has been unwavering and his contributions are extensive.
“The instrumental role he played in Knighthead becoming involved in Birmingham City FC should also never be forgotten. We are delighted that he will remain connected to us via the Advisory Committee and Foundation and that both will benefit from his vision and expertise.”
Jeremy Dale is assuming the role of interim CEO, effective immediately.
Mr Wagner continued: “Jeremy has a deep understanding of our vision and culture, and we are confident in his ability to guide Birmingham City during this transition.
“Our commitment to excellence and progress remains steadfast as we continue to build on the solid foundation we’ve already established.”
