When Brown took office -- by stepping in as Labour leader when Blair stepped down in 2007
-- he promised to win back people's trust in politicians. But his tenure was marred by a failing economy, a difficult and bloody war in Afghanistan, and a scandal over outrageous expense claims filed by lawmakers.
He also had the misfortune to follow one of Britain's most mesmerizing leaders.
"He's a victim of the fact that he comes after Blair," said Victoria Honeyman, a lecturer in British politics at the University of Leeds. "Blair is hugely charismatic. Even people who didn't vote for Labour thought Tony Blair would be a nice guy to have a drink with. Coming after Tony Blair drives it into even sharper focus that he's not that person."
Brown may not disappear from the spotlight immediately. He could remain in Parliament after he steps down as Labour leader, as former Conservative Prime Minister John Major did. Or, like Blair, he could quit soon afterwards.
Some have suggested he might like a role at the International Monetary Fund or World Bank.
Brown himself has signaled he sees his future outside of politics.
"If I couldn't make a difference anymore, I'd go off and do something else," he said during the election. "And Sarah and I might do charity, voluntary work. I don't want to do sort of business or anything else, I just want to do something good."
Brown's sometimes awkward demeanor was fodder for satirists, and a recent run-in with an elderly voter
-- whom he called a "bigoted woman" -- certainly wasn't smooth. But behind closed doors, Brown, 59, was often described as warm and agreeable.
The qualities he is best known for are sheer grit and perseverance.
"He doesn't possess charm or eloquence, but he does possess a sort of dogged resilience, an ability to plug on and not give up
-- in a way it enabled him to outlast all his critics," said Bill Jones, a professor of politics at Liverpool Hope University in northern England.
"He's not been a very popular (prime minister) with voters and with his colleagues, but most people in the end would allow some admiration for those bulldog qualities."
Brown was born in the Scottish industrial town of Kirkcaldy on Feb. 20, 1951. His father was a Presbyterian minister, a profession Brown has said inspired his sense of moral mission.
He entered the University of Edinburgh when he was just 16, and at 21 was elected rector of the university
-- a largely ceremonial leadership post. Brown had suffered detached retinas in both eyes while playing rugby, and lost the sight in one eye permanently, but surgery saved vision in the other. His university years coincided with periods in hospital or recovering, but he still managed to graduate with top honors.
He was elected to Britain's Parliament on his second attempt in 1983, rose quickly in the Labour Party and was tipped to be its next leader when party chief John Smith died of a heart attack in May 1994. But it was Blair who ran for the leadership, after the two men reputedly struck a deal at a London restaurant for Blair to become prime minister, with Brown in charge of the economy.
After Labour's landslide election victory in May 1997 -- which ended 18 years of Conservative rule
-- Brown placed his stamp on economic policy. One of his early moves was to hand power to set interest rates to the Bank of England. He also set a series of economic criteria for British entry into the European single currency, effectively ensuring Britain would not join the euro.
But Brown grew increasingly restive as the years passed without any sign of Blair moving on.
