Father Wulstan Peterburs OSB has been appointed interim headmaster after it was revealed David Lambon was leaving his role at Ampleforth College in North Yorkshire.
In a letter to parents and guardians, Ampleforth College said the decision was taken following "careful consultation" with the school's governing body, St Laurence Education Trust.
David Lambon embarked on a new far-reaching development plan for Ampleton with "an emphasis on seeking to raise academic results even higher, whilst ensuring the fullest and best possible holistic education for our young people in the context of our Catholic Benedictine ethos", Ampleton College said.
The institution said David Lambon had "built impressively upon the achievements of his predecessor", adding: "Leading his senior team, David has concentrated his and the College's efforts into a new development plan, covering the full range of school life from the academic to the pastoral.
"This includes, but is not limited to, the artistic, musical, social and sporting, with an emphasis on seeking to raise academic results even higher, whilst ensuring the fullest and best possible holistic education for our young people in the context of our Catholic Benedictine ethos, which informs and influences all that we do."
Fr Wulstan, who read a PhD in theology at the University of Durham, joined Ampleforth College in 1994, before going on to serve as Head of Christian Theology.
Abbot Cuthbert Madden, the Chair of Governors from Ampleforth College, said: "I have asked Fr Wulstan Peterburs to take over the responsibilities of Headmaster, as we believe him to be best placed to maintain our future development."
Commenting before he took up the post in September 2014, David Lambon said: "It is a real privilege to be asked to be Ampleforth's first lay head.
"Having already seen the strength of Ampleforth's Benedictine ethos, my role will be to build on this, and the school's many other strengths.
Abbot Cuthbert Madden said at the time: "The recruitment of the College's first professional lay Head was a big step, but in David we are confident we have found an exceptional teacher and leader with a strong record of commitment to, and achievement in, Catholic education, who will work with the community to retain the school's Benedictine character, and will take the College to new levels of success."