A strong advocate of devolution, the former Scottish Secretary, Dr John Reid (53), who has been propelled into the office of Northern Ireland Secretary, is well regarded at Westminster as an able, efficient member of the cabinet.
The Motherwell-North MP, who has held his seat since 1987, is renowned for his humour and wit. He has emerged through the parliamentary ranks, having been Armed Forces minister, he took over at the Department of Transport in 1998 before going to the Scottish Office the following year, and now the Northern Ireland Office.
He was born a Catholic and "went through an atheist period" according to his parliamentary profile and he has also formed a Celtic supporters' club at Westminster.
He was described by one journalist as "one of the few MPs from the Glasgow area who can speak English and is therefore comprehensible to the rest of us".
As Armed Forces minister, Dr Reid met campaigners demanding the release of the two Scots guards convicted of the murder of the Belfast teenager, Mr Peter McBride.
He also consistently resisted calls for an independent inquiry into the circumstances of the Chinook helicopter crash on the Mull of Kintyre in 1994 which killed several high ranking members of the RUC and British intelligence officers.
Dr Reid, who is a widower, has two sons.
Last night Sinn Fein party chairman Mr Mitchel McLaughlin said: "While I wish John Reid well in his new appointment Sinn Fein understands that irrespective of who the British Secretary of State is what is important is British policy. And British policy at this time is enshrined in the Good Friday Agreement."
"We will be looking to Mr Reid to promote, defend and act as guarantor of that agreement."
Ahead of the announcement of Dr Reid's arrival in the Northern job, speculation focused on Mr Mandelson's former minister at the Northern Ireland Office and now Welsh Secretary, Mr Paul Murphy, being named as the next Northern Ireland Secretary.
Another name in the frame was the current number two at the Northern Ireland Office, the Northern Ireland Security Minister, Mr Adam Ingram. At one stage the junior Northern Ireland Minister, Mr George Howarth, was also seen as a possible successor.
When he was a Northern Ireland minister, the Welsh-born Mr Murphy (52) assumed responsibility for political development at Stormont during the protracted negotiations ahead of the signing of the Belfast Agreement.
It was generally regarded at Westminster that Mr Murphy, who is the MP for Torfaen, would not have much difficulty assuming the reins at the Northern Ireland Office because of his previous experience.
The other contender, the East Kilbride MP, Mr Ingram (53), also has considerable relevant experience. His solid Commons performances during the passage of the Police (Northern Ireland) Act through parliament point to a careful understanding of the detail of Northern Ireland's difficulties.
Certainly the fact that he is already a Northern Ireland Minister deeply involved in the minutiae of the talks process and has a clear grasp of the positions of all the parties weighed in his favour.
He has had responsibility for security issues and prisons at the Northern Ireland Office since his promotion from the back benches after Labour's election victory in 1997 and he is a skilful performer in the House of Commons.
In the Commons during Northern Ireland Questions yesterday, Mr Ingram was typically robust, displaying the no-nonsense attitude that often makes the Opposition sit up and listen carefully.
As Mr Mandelson sat pondering his future, Mr Ingram vigorously deflected Conservatism criticism of the British government's handling of policing by reminding the Opposition of the spirit of bipartisanship.
Dr John Reid has been mentioned with regard to the North before - he emerged as a possible successor to Dr Mowlam before the Prime Minister gave Mr Mandelson the post.
Ahead of the announcement of Mr Mandelson's successor, the Ulster Unionist MP, Mr Ken Maginnis, preferred not to guess who might succeed Mr Mandelson, but said an "unduly volatile" character would be unwelcome. "It has to be someone who brings the same degree of commitment and logic to the situation as Peter Mandelson," he said.
"Somebody who has the same degree of attention to the safety of the people of Northern Ireland. Peter Mandelson could be volatile at times, but he could also be calmed down.