Sir Laurie Magnus was named as the new independent adviser on ministers' interests on Thursday.
His predecessor, Christopher Geidt — in position as consequences of the Partygate controversy became felt — resigned a month after saying Mr Johnson must explain why he thought he had not broken the ministerial code after being fined over attending a party during a Covid-19 lockdown.
Since then, Mr Johnson has been forced out of office, Liz Truss was named his successor only to be ousted over calamitous budget plans, and now Mr Sunak is in No 10 insisting on “integrity, professionalism and accountability at every level”.
Mr Magnus, the chairman of the Historic England charity, will act as independent adviser on ministers' interests.
He has more than 40 years' experience in the financial services industry, with expertise in auditing, compliance and corporate governance.
Since 2013, he has led Historic England, a public body that helps preserve places of historical interest. He is also a former deputy chairman of the National Trust.
In a letter to Mr Magnus, Mr Sunak said any independent adviser played a critically “important role” in government.
“I have sought to identify potential candidates who can demonstrate the critical qualities of integrity and independence, relevant expertise and experience, and an ability to command the trust and confidence of ministers," he wrote.
“Having discussed the role with you, I am confident that you not only demonstrate these qualities but that you will serve in the role with distinction, in the best traditions of public service.”
Mr Sunak, who became Prime Minister in October, said filling the role had been a priority to help ensure standards were upheld and the public could have confidence in the government.
In a letter to the Prime Minister, Mr Magnus said an “early priority” would be to scrutinise the declarations of interests by government ministers.
“I will endeavour to discharge the important responsibilities of the role with fairness and integrity, in a manner which inspires the confidence of ministers, Parliament and the public,” he told Mr Sunak.
“I see maintaining that confidence as a critically important element of governance in this country. It is an honour to be asked to carry out the role, and a significant responsibility.”