Her exit from Castle Trust comes just days after the lender’s founder and chief executive Sean Oldfield announced he is stepping down.
Upton joined the lender in 2013 and will remain in the post until a successor has been appointed.
She said: “I’m very proud of the team that we’ve built here at Castle Trust and am sure that the achievements of the business will continue as it transitions to become a bank.
“It has been a difficult decision to make, however knowing that I leave the mortgage division in the hands of some of the best people in the industry makes it an easier decision to step aside from the established business that Castle Trust has become.”
Oldfield founded Castle Trust in 2011 with backing from private equity firm J.C Flowers and this month the firm announced its intention to apply for a banking licence.
Oldfield is to remain on the lender’s board as a non-executive director, and is replaced as chief executive by Martin Bischoff, who joins from Santander.
Oldfield last week said: “I am incredibly proud of building Castle Trust from scratch into a substantial business today.
“As we prepare to move from being a speciality finance provider into a bank, it is an appropriate moment for me to consider a new challenge for the next stage in my professional career.
“I am looking forward to continuing to play a part in Castle Trust’s development as a non-executive director.
“I am also keen to maintain the very strong partnership that I have built with J.C. Flowers & Co., who have been such supportive shareholders and business partners in the growth of Castle Trust and are underpinning the company’s move to become a bank.”
Castle Trust now has more than £750m in assets and 200 team members.
Tim Hanford, managing director in charge of J.C. Flowers & Co. Europe, said: “Sean is a natural entrepreneur and we understand his desire to start a new venture.
“We wish him every success with his new business and look forward to continuing to benefit from his experience on the Board. Looking forward, we are excited about Martin Bischoff joining the team, and have every confidence that he will build on the existing platform to take Castle Trust to continued success.”