By Ian Graham | BELFAST
The face of Irish Republicanism for many during some of the worst moments of three decades of sectarian bloodshed that killed more than 3,600 people, McGuinness remained a figure of hate for many pro-British Protestants until his death.
But he earned widespread respect across Britain and Ireland by embracing his bitterest rivals to cement the 1998 peace deal and allow Northern Ireland to slowly return to normality.
"While I can never condone the path he took in the earlier part of his life, Martin McGuinness ultimately played a defining role in leading the Republican movement away from violence," British Prime Minister Theresa May said.
"In doing so, he made an essential and historic contribution to the extraordinary journey of Northern Ireland from conflict to peace."
Irish Prime Minister Enda Kenny said McGuinness strove to make Northern Ireland a better place for everyone, regardless of background or tradition.
McGuinness was present during the opening salvoes of the conflict as a 20-year-old IRA commander fighting the British army on the streets of his native Londonderry on behalf of a community he said had been denied basic human rights.
He swiftly rose to become a senior IRA commander and was convicted in 1973 of being a member of the group after being stopped in a car packed with explosives and bullets.
"Martin McGuinness never went to war, it came to his streets, it came to his city, it came to his community," fellow Republican leader Gerry Adams told Irish national broadcaster RTE.
"Martin led the IRA when there was a war but he led the IRA into peace. He was a great man in my opinion and he will be missed."
By the 1980s McGuinness emerged alongside Adams as a key architect in the electoral rise of Sinn Fein, the IRA's political wing, advocating a strategy of using the ballot box alongside the Armalite rifle.
John Major, who was British Prime Minister when the IRA declared a ceasefire in 1994, said that while McGuinness's actions in the early part of his life were unforgivable, subsequent brave decisions by him saved many lives.
"When he approached us, making it clear, in effect, that he was prepared to contribute to a peace process, he was taking a risk. I had difficult backbenchers who opposed what I was doing, but he had difficult volunteers who were much more violent than my backbenchers," Major told Sky News.
Following the IRA's second ceasefire in 1997, McGuinness became Sinn Fein's chief negotiator in peace talks that led to the landmark 1998 Good Friday peace accord.
"He had the grassroots credibility of a Republican leader and former IRA commander ... to take Republicans from the past of terror and horror into a democratic future," former British Northern Ireland minister Peter Hain told BBC radio.
But it was the energy with which McGuinness worked to bed in the peace process that surprised many. His handshake with the British Queen in 2012 became one of the defining images of Northern Ireland's peace.
Key to the success of power-sharing in Northern Ireland was the close relationship with former enemy Ian Paisley, the firebrand preacher who for decades brayed "No Surrender!" across Northern Ireland's sectarian divide.
A partnership many thought would prove impossible was soon dubbed by the media "the Chuckle Brothers" and allowed McGuinness to become Northern Ireland's Deputy First Minister in 2007. He held the role for a decade until he resigned in January shortly after being diagnosed with a rare heart condition.
On the night McGuinness retired, Paisley's son Ian Junior, a member of the British parliament, said had it not been for McGuinness' work, especially with his father, Northern Ireland would not have been in a position to rebuild itself.
Mourning McGuinness, Paisley Junior said it was not how you start your life that was important, but how you finish it.
McGuinness was active until the last weeks of his life, helping to orchestrate one of the biggest political victories for Irish nationalism in decades by forcing a snap election in March that deprived unionism of its majority in the regional parliament for the first time.
Former first minister Arlene Foster, whose party was humiliated in the vote and whose father narrowly escaped alive from an IRA shooting, also paid her respects.
"History will record differing views and opinions on the role Martin McGuinness played throughout the recent and not so recent past but history will also show that his contribution to the political and peace process was significant," Foster said.
Some were less forgiving. Former conservative minister Norman Tebbit, whose wife was badly injured in an IRA blast in 1984, said the world was "a sweeter place" without McGuinness, who he described as a coward posing as a man of peace.
McGuinness leaves Northern Ireland at peace and his dream of a united Ireland inching closer under a new generation but with its politics still divided. The squabbling parties have just days to resurrect the power-sharing government.
One former rival said McGuinness' negotiating skills would be sorely missed.
"There are many today, as we sit with the clock ticking down, who think that if you were at the helm, we would face this prospect with greater optimism," former unionist first minister David Trimble wrote in a recent letter to McGuinness published by Irish national broadcaster RTE.
(Additional reporting by Estelle Shirbon, Alistair Smout, writing by Padraic Halpin and Conor Humphries; Editing by Andrew Heavens and Ed Osmond)