Rev Martin Smyth (94) served as MP for South Belfast from 1982 until 2005.
The Co Down man took the seat after Robert Bradford was murdered by the IRA in November 1981.
Mr Smyth also served as Grand Master of the Orange Order for 26 years.
Ulster Unionist leader Mike Nesbitt led tributes.
He said: “I am terribly sorry to hear of the passing of Martin Smyth.
“From my days as a broadcast journalist, he was an ever-present heavyweight in local politics, whose views were always important.
“He may not have been as prominent as others in the public sphere, but hugely influential behind the scene.
“He was another of that fateful generation of what we so euphemistically call ‘The Troubles’ whose imprint will last forever.”
DUP leader Gavin Robinson also offered his condolences.
The East Belfast MP said: “My sincerest sympathies to the family of the Rev Martin Smyth. A stalwart of unionism, he stepped forward in South Belfast at a most volatile time.
“Firm and faithful, he represented his constituents and NI with a strong sincere strength and unswerving belief in the Union.”
TUV leader Jim Allister said Rev Smyth “served Protestantism and unionism for many years through some of the worst of the terrorist campaign”.
The Grand Orange Lodge of Ireland described Rev Smyth as “a leader of unwavering principle and determination through often difficult times and a stalwart of the Orange cause”.
Former First Minister Arlene Foster reminisced on the “happy memories” she shared with Mr Smyth.
Jim Nicholson, previous UUP Member of the European Parliament, said his colleague was “a titan” of unionism.
“He will be remembered for his quiet confidence but also his power as an orator and champion of Ulster unionism,” Mr Nicholson added.
Lord Peter Weir, a former Education Minister, said he was “a staunch unionist and Orangeman, dedicated public servant and man of conviction”.
“He was prepared to be a political representative at a time when it literally risked his life,” he added.
Rev Smyth resigned his ministry and was elected to Parliament in the 1982 by-election following the murder of Rev Bradford.
Rev Bradford (40) and a 29-year-old caretaker, Ken Campbell, were shot dead during a constituency clinic at Finaghy Community Centre. No one has ever been convicted of the murders.
In a lengthy political career, Rev Smyth was an unsuccessful candidate for the UUP leadership in 1995 after the resignation of Lord Molyneaux, finishing fourth to the winner, David Trimble.
In January 2005, Rev Smyth announced he would be stepping down from Westminster at the next election to spend more time with his wife.
Alasdair McDonnell took the South Belfast seat for the SDLP.
UUP councillor Andrew Wilson extended his sympathies to Mr Smyth’s loved ones, adding: “I had the pleasure of spending time with Martin at the Somme in 2013. My thoughts are with his family circle and many friends at this sad time.”
Bradley Ferguson, a DUP councillor for Lisnasharragh, paid tribute to the “man of deep faith.”