Naoto Kan faced the toughest test of any Japanese prime minister for decades.
The biggest earthquake in the country's history, which triggered a devastating tsunami, followed by the worst nuclear accident since Chernobyl.
But after little more than 14 months in the job, he was forced out of office because the opposition and many in his own party believe he failed to show sufficient leadership in the crisis.
Survivors of the deadly 11 March natural disasters in the country's north-east have complained of the slow recovery response.
Mr Kan was said to be too slow in acknowledging the severity of the disaster at the Fukushima plant, where radiation is still leaking from the crippled reactors.
The 64-year-old staved off a no confidence motion in June only by promising to resign once key bills on the budget and renewable energy passed parliament.
A founder of the Democratic Party of Japan (DPJ) that swept the LDP from power in September 2009, he inherited a divided parliament, a stagnating economy and massive public debt.
He leaves office amid plummeting public support and political infighting - including within his own party, and with Japan's national debt the biggest in the industrialised world and ballooning.
Naoto Kan was something of a rarity in Japanese political circles - he is not from the political elite.
Nor has he ever been a member of the Liberal Democratic Party (LDP), which governed the country for more than half a century.
He was born in Yamaguchi in the south, the son of a factory manager.
A physics graduate from the Tokyo Institute of Technology, he ran a patent firm and then became a civic activist, focusing on environmental issues.
In 1980 - after three unsuccessful attempts - he won a seat in parliament as part of the tiny Social Democratic Federation.
He served as health minister under a coalition deal with the LDP in the mid-1990s, becoming very popular after he exposed a scandal involving tainted blood products.
He forced bureaucrats to release documents which showed the government had failed to prevent the use of HIV-infected blood products for transfusions.
The scandal provoked a public outcry, and his handling of it propelled him high in opinion polls.
He went on to co-launch the DPJ and led it during the election of 2003, establishing the party as a credible opposition force and potential challenger to the LDP.
But a year later he stood down, after admitting that he had failed to make state pension payments for 10 months while he was health minister.
Although his failure to pay was an apparent oversight, Mr Kan said he was resigning to avoid further damaging the DPJ.
Five years later, he became deputy prime minister when Yukio Hatoyama swept to power amid widespread popular discontent with the LDP government.
He was also appointed to head the National Strategy Bureau, a new body charged with wresting control of policy-making from the powerful bureaucracy.
But he took over the finance portfolio four months later after Hirohisa Fujii stepped down due to ill health.
Announcing his candidacy to replace Yukio Hatoyama in June 2010, he emphasised his ordinary roots.
"I grew up in a typical Japanese salaryman's family. I've had no special connections," he said.
"If I can take on a major role starting from such an ordinary background, that would be a very positive thing for Japanese politics."
Popular because of his unorthodox route to the top, he was known as a keen debater. His nickname is "Ira-Kan" - ira being short for irritable - because of his reportedly quick temper.
With just three months under his belt as prime minister, Mr Kan survived a leadership challenge by DPJ veteran powerbroker Ichiro Ozawa.
Mr Kan also received a stinging rebuke by the electorate last July, when the DPJ lost their majority in the upper house of parliament over proposed tax reforms.
Married with two sons, he enjoys playing Go, a complicated chess-like board game.