Still, the president added: "We've got a lot of work to do."
Appearing at the campaign-style event in Arnold, Mo., Mr. Obama told the crowd he was "pleased with the progress we've made but I'm not satisfied."
"I'm confident with the future but I'm not content with the present," the president added.
In office just three months, the Democrat enters the next phase of his new presidency with a high job approval rating and a certain amount of political capital from his history-making election last fall. But he also faces a thicket of challenges as he seeks to move ahead on multiple fronts both foreign and domestic amid recession and war. He will need continued public support to accomplish his lofty goals.
Thus, Mr. Obama used the anniversary - some aides derided it as a "Hallmark holiday" - to press his case.
He defended his ambitious, costly plan, saying: "These challenges could not be met with half measures. They couldn't be met with the same, old formulas. They couldn't be confronted in isolation. They demanded action that was bold and sustained."
Mr. Obama also took the opportunity to counter critics who said he's taking on too much, as he works to turn around the recession while revamping energy, education and health care in the United States.
"The changes we made are the changes we promised," Obama said. "We're doing what we said we'd do."
Mr. Obama began his day officially welcoming Sen. Arlen Specter, the veteran Pennsylvania Republican, into the Democratic party - a development the president said he was "grateful" for. Vice President Joe Biden, who had long encouraged his former Senate colleague to become a Democrat, also attended.
The president then left for the town hall meeting before planning to return to Washington in the afternoon to prepare for a prime-time news conference, his third since taking office.
Darrell West, director of governance studies at the Brookings Institution, said the White House is smart to try to take control of the message on the 100th day.
"It's always good to play offense," West said. "If you create a vacuum, someone else is going to fill it."
Like most of the 12 states Mr. Obama has visited in his first 100 days, Missouri is politically important. A traditional bellwether in presidential elections, Missouri went to Mr. Obama's opponent, Republican John McCain, by just a few thousand votes in 2008. Not only will Mr. Obama be eyeing Missouri in 2012, but Democrats see an opportunity to pick up another Senate seat there in 2010 when long-serving Republican Sen. Christopher Bond retires.
Playing up to the crowd, the president told the crowd it was "great to be back in Middle America, where common sense reigns."
Fielding questions about the struggles of U.S. automakers, Mr. Obama said it isn't clear whether Chrysler LLC will be able to meet the government's Thursday restructuring deadline.
He urged Chrysler's bondholders to "make sacrifices" to help keep the company out of bankruptcy protection. Mr. Obama also said that the automaker's unions have made "enormous sacrifices, enormous sacrifices to try to keep the company going" and it is now up to Chrysler's creditors to do the same.
On education, President Obama said the U.S. must convince students it's "cool to be smart" as he expressed worry that the country is losing its love of learning.
Mr. Obama said that parents and communities can help foster that attitude by paying as much attention to science award winners as they do to basketball stars, for instance.
Mr. Obama's efforts to fix the nation's economy have drawn comparisons to President Franklin Roosevelt, who is largely responsible for the 100 days phenomenon. Roosevelt launched many of his New Deal programs during that period and, with backing from Congress, signed 15 major bills into law.
"There was a strong bipartisan effort in Congress to follow the president's lead," said David Woolner, senior vice president of the Franklin and Eleanor Roosevelt Institute.
Mr. Obama hasn't had that support. His biggest legislative accomplishment - the $787 billion economic stimulus bill - passed Congress with scant Republican backing.
Woolner said the political parties have changed since Roosevelt's day, making it difficult for presidents to achieve the same level of bipartisan support Roosevelt enjoyed. Still, he said, the first 100 days remains a relevant benchmark.
"Presidents who have been successful in this period have done a good job in setting an agenda for the coming years," Woolner said.
As the president spoke, the House on Wednesday that endorses much of his ambitious agenda while permitting the national debt to continue to spiral higher.
While a welcome victory for Democrats, the 233-193 House vote approving the House-Senate compromise budget is only a first, relatively easy step toward Mr. Obama's goal of providing health care coverage for all Americans. Next would come arduous negotiations among lawmakers, the Obama administration and a vast array of interest groups.
Not a single Republican voted for the measure; 17 Democrats, mostly from GOP-leaning districts, voted against it.
The Senate will vote on the measure Wednesday afternoon.
While Mr. Obama may lack bipartisan support, he reaches his 100th day with strong public backing. A CBS News/New York Times poll found that of Mr. Obama's job performance and 41 percent believe the country is headed in the right direction. The "right direction" number is up 18 points since February and 34 points since October.