New Zealand native Mr Brown joined Ulster Bank in April 2011 and was tasked with guiding it back to profitability in the wake of the banking crash.
He is to remain within Ulster Bank’s parent group, RBS, for the time being, taking up the role of chief executive at Williams & Glyn.
The new UK challenger bank, which is made up of more than 300 branches across the UK, is due to list on the London Stock Exchange in the latter part of 2016.
As part of the UK government’s bailout of RBS, European regulators ordered RBS to sell Williams & Glyn.
“Jim Brown brings a wealth of experience in retail and commercial banking and will lead the business as we move forward towards an IPO in the second half of 2016,” said RBS CEO Ross McEwan yesterday.
The outgoing Ulster Bank chief executive replaces current CEO John Maltby, who is to resume his role as an adviser to an external advisers’ consortium.
“I am pleased to have been appointed to the CEO role at what is an exciting time for the business,” said Mr Brown. “We are committed to establishing a strong and credible challenger bank in the UK market and I look forward to the challenges ahead.”
In February, Ulster Bank announced its return to profitability for the first time since 2008.
The bank reported €752m for the year to the end of December 2014, having been able to write back more than €450m of impaired loans during the year.
Rising property values and more favourable economic conditions also helped its cause.
Along with the country’s other main lenders, the bank has been criticised for failing to do enough to tackle the mortgage arrears crisis and failing to pass on lower interest rates to customers.
Earlier this week, Ulster Bank announced measures aimed at some 2,000 borrowers in mortgage arrears but not engaging with the bank.
The measures include a commitment that customers’ homes will not be repossessed if revised repayment terms are agreed.