The Prime Minister has other plans for him in the meantime. Mr Finlayson's ability to keep a secret will come in handy in his Minister of Spies role: he has even kept secrets from Mr Key before.
Mr Key made Mr Finlayson responsible for the Security Intelligence Service and Government Communications Security Bureau while the PM retains an overarching responsibility as Minister of National Security.
The split means Mr Finlayson will have day-to-day responsibility for the two intelligence agencies, including signing warrants when the GCSB is asked to spy on New Zealanders on behalf of the SIS or police.
It is a job in which secrecy is required.
But his role as Attorney-General also involves keeping secrets; he even has a motto: "The more you know, the less you say."
He was told of the FBI's interest in Kim Dotcom in November 2011 - two months before the Prime Minister was told by the Solicitor-General on the eve of the raid on Dotcom's Coatesville mansion.
Mr Finlayson insists he did not discuss it with Mr Key before then, despite the scepticism about Mr Key's claims not to know.
"I learn a lot of things as first law officer of the Crown that I don't share with other people."
Mr Finlayson said he was yet to receive his briefing on his new portfolios, but one of his tasks next year will be overseeing the first independent review of the spy agencies - a review demanded by United Future leader Peter Dunne as a condition for his support for the new GCSB legislation last year.
Mr Finlayson said the split in responsibilities between himself and the Prime Minister was not unique. In Australia, the Attorney-General, George Brandis, was also Minister responsible for the Australian Security Intelligence Organisation.
Although Labour has accused Mr Key of trying to wash his hands of responsibility for the agencies at a time when they are politically contentious, Mr Finlayson dismissed his rivals' objections as "purple prose".
"Objective analysis indicates this kind of movement of responsibility is not unheard of, and it is a good thing to separate out the overall policy from the day-to-day responsibility for the departments."