"Scotland's Parliament is no longer a political pamphlet, a campaign trail, a waving flag," declared the weary-looking 61-year-old Glasgow solicitor, so obviously relieved that his lifetime's battle had been won.
Mr Dewar was clearly moved by the occasion. "This is a privilege which carries a great responsibility," he said, after shaking hands with the main party leaders.
He promised government based on co-operation, and repeated what has been the recurrent theme of these opening days: that the new Parliament should not become an elitist club. "We must look beyond the walls of this place to the people of Scotland," he said.
His election to the post of First Minister had been contested by Dennis Canavan, the left-wing independent, and two of the other main party leaders, David McLetchie of the Tories and Alex Salmond of the SNP. Only Jim Wallace, leader of the Liberal Democrats and contemplating a cabinet seat in coalition with Labour, stood back from the fray.
Opponents taunted his party for backing Mr Dewar before a formal coalition had been agreed. "They're buying a pig in a poke," said Mr Canavan, as Mr Wallace squirmed.
Yet for a few moments after the vote, the house shared "Donald's" achievement.
Voting for this master fixer, who excelled as a chief whip in Westminster, had gone almost perfectly to Labour's plan. All but one of the Liberal Democrats' 17 members had rallied behind him as his own party stood full square, securing for him 71 of the Parliament's 129 votes.
There was a dizzy lightness about the occasion among many members still new to the whole process of parliamentary government. The electronic voting system, more familiar to game-show contestants than politicians, confused even the most experienced. Amid laughter, Winnie Ewing, the oldest member of the house and a veteran of the European Parliament, was forgiven for pushing the wrong button.
Mr Salmond, the SNP leader, promised a break with the "yahoo" politics of the past. Mr McLetchie, for the Tories, recognised Mr Dewar's lifetime achievement and joked that he wished his rival a swift and happy retirement.
And then the acrimony and bitterness of politics punctured the moment.
Mr Canavan, who was expelled from the Labour Party only to be returned with a huge majority as an independent, rose to his feet. "I know that Donald Dewar said I was not good enough to sit in this Parliament," he declared, harbouring old hurts. "I only hope that in four years he proves himself to be good enough to be Scotland's First Minister."
Mr Canavan then strode dramatically across the floor of the chamber to shake the First Minister's hand and "show I bear him no malice". Mr Dewar could hardly have looked more alarmed if Mr Canavan had struck him.