Stephenson has joined the trade association following time spent as special adviser to former secretary of state Andrew Lansley at the Department of Health before September’s reshuffle.
PRWeek understands he has stepped into the newly created role of executive director, external affairs, at the BBA.
The appointment comes as scrutiny of the banking industry continues, with Barclays hitting back this weekend over US regulators' proposed fines of $470m (£291m) for the bank's alleged manipulation of energy markets.
Stephenson said: ‘I'm honoured to be joining the BBA at this critical time for the banking sector in the UK. The BBA has a leading role to play in pushing for reform and ensuring the banking sector meets the needs and expectations of both customers and shareholders.
‘Banking is one of our most important industries. It plays a pivotal role in our economy and employs hundreds of thousands of British workers. I'm looking forward to the challenge of playing my part in helping the BBA to restore the public's trust in this crucial industry once again.’
When Lansley was health secretary, he led controversial reforms to the NHS that are due to come into force in this spring, placing GPs at the centre of commissioning services. He was replaced by Jeremy Hunt in Prime Minister David Cameron’s September reshuffle, and moved to become Leader of the House – widely seen as a demotion.
PRWeek revealed earlier this month that Lansley spad Bill Morgan had returned to MHP as senior adviser.