John George Lambton, the first Earl of Durham, is perhaps the most famous of men who have held this title. He is widely regarded as the architect of responsible government in the Canadas, which eventuated in Confederation; his Report on the Affairs of British North America is by general consensus one of the most influential documents in Canadian history.

Born on April 12, 1792 in London, Lambton was the son of a member of the House of Commons. Following his father into politics, he became a moderate reformer in the Whig party, earning himself the nickname "Radical Jack" along with the cautious respect of his contemporaries.

In autumn of 1813 he was elected to the House of Commons as the representative for Durham; when the Whigs finally came to power in 1830, under Lord Grey and as Lord Privy Seal, he was appointed leader of a committee to reform parliament. This resulted in the reform bill of 1832, and also saw his popularity increase; now, instead of merely worryingly radical, his reputation had added to it some tangible success.

By way of reward for his work on the reform bill, he was appointed ambassador to Russia in 1835. This posting lasted two years; upon his return to England, he was offered another overseas station, this time in the colonies of British North America which were at that time embroiled in political unrest and seemingly unsolvable turmoil.

Wisely, he declined.

After the first rebellion in the Canadas had run its course, Durham was again approached by the British government with the same offer, only this time with a twist: he was informed that in his capacity as Governor General and High Commissioner of British North America, he would have almost unlimited power as a colonial dictator of sorts, and essentially all that he would be required to do in return was prepare a report detailing the origins of the rebellions.

Power such as this was an offer that was difficult if not impossible to refuse. Understandably, this time Durham accepted. He set off for the Canadas shortly thereafter, and landed in Quebec on May 27, 1838.

At this point, the rebellions were by no means at an end. The underlying problems that had led to the outbreak of violence in 1837 had not been solved, and tensions were running high, particularly in Lower Canada where a number of men involved with the insurrection had been taken prisoner and were being held pending execution.

Durham's actions, in line with the power he had been given, were quick and decisive. Rather than executing those prisoners who had been particularly instrumental in the rebellions, he had them sign a statement declaring their guilt in return for safe passage to Bermuda; the rebels who had fled to the United States, among them Louis-Joseph Papineau, were forbidden to re-enter British North America, and the rest of the prisoners were set free.

This was hardly in line with the expectations held by the Crown for his conduct. Within six months of his landing in Quebec, Durham resigned from his post and was brought back to England in disgrace.

Once away from the immediacy of violence in the colonies, he was able to prepare a report relating to the causes of the rebellions. It was presented in London on February 11, 1839. In it he asserted that the cause of the unrest lay in racial differences rather than in politics; naturally, the upper-class Anglophone Tories hailed it as genius, while the Francophone politicians in the Canadas were outraged.

Rather than defend his report and the contentions therein, Lord Durham died on July 28, 1840. It wasn't until some years following his death that the suggestions made in the report were put into effect and found to be surprisingly successful, laying the foundations for a system of truly responsible government and national unity.

Sources:
http://www.nationmaster.com/encyclopedia/Lord-Durham
http://www.collectionscanada.ca/2/18/h18-2086-e.html
Ouellet, Fernand. Lower Canada 1791-1840: Social Change and Nationalism. Toronto: McClelland and Stewart, 1980.