Traditionally, commentators have used the 100-day milestone to assess presidents' early successes and failures.
At a news conference broadcast live on most US TV networks, the president said that his first 100 days in office had been successful, but freely admitted more needed to be done.
I am surprised... by the number of critical issues that appear to be coming to a head all at the same time
"I'm pleased with the progress we've made but I'm not satisfied," Mr Obama said.
"I'm confident with the future but I'm not content with the present."
He told reporters he had not anticipated finding himself in the midst of the worst economic crisis since the Great Depression.
"I am surprised, compared to where I started, when we first announced for this race, by the number of critical issues that appear to be coming to a head all at the same time," he said.
'Broken system'
Mr Obama took questions on many of the key issues his administration has begun to grapple with, both at home and abroad.
Among his responses he said that:
Pakistan's government was "very fragile", but he was confident that the country's nuclear arsenal would not fall into militant hands
A series of deadly bombings in Iraq were a cause for concern, but the incidence of such attacks remained much lower than last year
Steps taken against harsh interrogation techniques used under the Bush administration would make the US stronger and safer in the long-term
The government should help troubled auto makers Chrysler and General Motors to stabilise, then end its involvement as quickly as possible
On immigration, the US could not continue with a "broken system", Mr Obama said, adding that he remained committed to comprehensive reform.
The US Congress capped Mr Obama's 100th day in office by approving a $3.4 trillion budget for 2010 - a move the president said would help to move the economy from recession to recovery.
We can't go back to an economy that is built on a pile of sand
Welcoming the news, the president said he was gratified that Congress passed the budget "so quickly and effectively".
"This budget builds on the steps we've taken over the last 100 days to move this economy from recession to recovery and ultimately to prosperity," he said.
But he also said that as the US tried to recover from the recession, it had to lay "a new foundation for growth."
"We can't go back to an economy that is built on a pile of sand - on inflated home prices and maxed-out credit cards; on overleveraged banks and outdated regulations."
"We have to lay a new foundation for growth."
Major emergency
Since taking office on 20 January, Mr Obama has, among other things, passed an economic stimulus package, ordered the closure of the prison camp at Guantanamo Bay, set a timetable for the withdrawal of US troops from Iraq and signalled a willingness to open up a diplomatic dialogue with countries like Iran and Cuba.
But as the president entered his 100th day in office, he faced a major domestic emergency in the form of the swine flu outbreak.
And the economic turmoil that has engulfed America since before Mr Obama entered the White House shows little sign of abating.
Apart from the stimulus package, Mr Obama has also attempted to shore up the economy with plans to recapitalise US banks and salvage failing car firms.
Mr Obama swept to victory in last year's presidential election with 53% of the vote.
The latest Gallup poll gives him an approval rating of 65%, virtually unchanged from his level of support on the day of his inauguration.
As he finishes the first phase of his presidency, Mr Obama is now expected to focus on passing his plan to give all Americans access to healthcare coverage, and introduce a cap and trade system to reduce carbon emissions and combat global warming.
Mr Obama's prospects of getting his ambitious agenda through Congress improved on Tuesday, when Republican Senator Arlen Specter announced that he was switching parties to become a Democrat.
If, as most people expect, the disputed Minnesota Senate race is decided in Democrat Al Franken's favour, Mr Obama's Democratic Party colleagues in the Senate will have the 60 votes they need to overturn any attempts by Republicans to block legislation.
WORDS TO DESCRIBE OBAMA
People were asked to describe President Obama in a word
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