The Affordable Care Act, widely known as Obamacare, is designed to offer insurance to millions of Americans who have never been able to secure it before -- some because of pre-existing health conditions like heart disease that insurers were unwilling to cover.
But its launch was hobbled by technical problems, casting doubt that the government could reach its 7 million enrollment goal by the end of March.
Since October 1, more than 2.1 million have secured coverage via Web portals set up by the federal government in 36 states and by local authorities in 14 others, Health Secretary Kathleen Sebelius told reporters in a conference call.
The bulk of the enrollment took place in December after a litany of glitches with the government website, healthcare.gov, which wasn't able to handle the full onslaught of users until the end of November.
The rocky start has played into the hands of Republicans who say the federal government has no business intervening in the private health care market and and should not be dictating health choices to Americans.
In addition, 3.9 million people found out via the sites that, due to the reform, they were eligible for coverage through public health insurance programs including Medicaid, which is geared toward the poorest, according to figures from October and November.
The government did not specify how many of the 6 million people total had previously not had health insurance -- a figure that will be key in determining the success of the reform deemed Obama's top domestic achievement to date. In particular, it was unclear how many simply renewed their coverage.
Some 50 million people currently do not have health insurance in the United States.
Those individuals who signed up for insurance before December 24 on the federal site will have coverage as of Wednesday, New Year's Day.
"Tomorrow, January 1st, will be a new day in health care for millions of Americans," Sebelius said.