Thorburn joined Clydesdale Bank in 1993 and took on the role of chief executive in 2011. He also held positions at Yorkshire Bank since 2004.
Thorburn commented on his departure:
I'm proud of the work my team and I have undertaken during my tenure as CEO, and I leave knowing that Clydesdale and Yorkshire banks are in much better shape.
However, having been CEO for almost four years, I came to the view that it was in the best interests of the business for me to stand down at this time and allow an injection of new leadership to take place.
As well as his leading role at Clydesdale Bank, Thorburn has served as a vice-chairman and chairman of the Confederation of British Industry in Scotland.
Clydesdale Bank board chairman Jim Pettigrew, commenting on Thorburn's exit, said:
David successfully completed the UK strategic review in 2012, which was undertaken to identify the changes needed to deliver sustainable and satisfactory returns from the UK banking business. As a result of David’s efforts, the UK banking business is in much better shape, with the 2014 results showing improvement in a number of areas including asset quality and a lower risk loan book
NAB is looking to sell off its UK lenders, the Yorkshire and Clydesdale Banks. Last year, NAB had to set aside £420m to pay for payment protection insurance (PPI) compensation claims.