Upton will report to Sean Oldfield, group chief executive at Castle Trust.
Wyles is leaving Castle Trust to take up a senior executive appointment in another financial institution, subject to regulatory approval.
Upton joined Castle Trust in 2013 as a broker development manager and was promoted to sales director, mortgages the following year.
Oldfield said: “Alex Upton is a rising star within Castle Trust and I am delighted that she has agreed to lead our mortgage business. Alex and her excellent team will play a pivotal role in realising our current three-year plan which will see us significantly grow our balance sheet during 2018 and beyond.”
Upton said: “I am thrilled to have the opportunity to lead the mortgage division. Castle Trust has always been a really exciting place to work and is a remarkable business. We have a tremendous opportunity to build something really special on the sound foundations which are already in place. I can’t wait to start.”
Marcus Dussard will succeed Upton as sales director after a promotion from business development director after joining Castle Trust in 2014.
Alex Hammond, the previous head of mortgage marketing at Castle Trust, left in mid-January to start a PR and content firm called Also Communications.