Unionists opposed to the Belfast Agreement have been unanimous in welcoming Mr Mandelson's resignation.
The deputy leader of the DUP, Mr Peter Robinson, said few tears would be shed over Mr Mandelson's departure from office. The Northern Secretary had consistently underestimated the level of support enjoyed by the anti-agreement DUP, he said.
"His refusal to recognise the fact that we speak for the majority of the unionist community characterised his time in office," he said.
Mr Robinson's party colleague, Mr Ian Paisley jnr, said his decision to publish a controversial letter in July 2000 had now been vindicated by Mr Mandelson's departure.
The Northern Secretary had criticised Mr Paisley's publication of a letter purportedly from the Northern Ireland Office. Mr Mandelson said the letter was a forgery.
"Last year Peter Mandelson wrote an open public letter condemning me for releasing to the media details of private correspondence that he claimed did not exist. I believe his resignation indicates that in the balance of probabilities that his statements then, as on Monday, are untruthful," Mr Paisley said.
The leader of the Northern Ireland Unionist Party, Mr Cedric Wilson, said he hoped Mr Mandelson's resignation would herald a new direction in government policy.
"There is now a window of opportunity for the Prime Minister to replace Mr Mandelson with a Secretary of State who is prepared to listen to the views of the majority of the unionist community," he said.
Mr Jeffrey Donaldson, the anti-agreement Ulster Unionist MP, also hoped the Prime Minister would reflect carefully on his choice of a successor for Mr Mandelson.
"With the deteriorating security problem and a crime wave on our streets, we need a Secretary of State who will take action to deal with the terrorist threat and with crime," he said.
Mr Donaldson added that Mr Mandelson's political epitaph would not be a happy one.
Mr Peter Weir, the UUP MLA for North Down, felt Mr Mandelson's term had brought little political progress.
"He was the epitome of spin over substance, and we have had too much spin and ambiguity in this process already," he said.
Mr Weir conceded, however, that there had been some positive aspects to the Northern Secretary's tenure.