The rate of people without health insurance changed little during 2012, dropping from 15.7 to 15.4 percent, according to the Census Bureau.
The tiny shift was attributed to a rise in people on Medicare, a trend that's expected to increase sharply as the baby boomer generation ages.
The total number of uninsured people dropped from 48.6 to 48 million in 2012, the report found. The rate of private healthcare coverage did not change significantly.
Federal health officials hope that as many as 7 million uninsured patients will sign up for the law's insurance marketplaces in 2014.
That figure could be as high as 8.5 million people in 19 states that are operating their own exchanges, according to a recent survey.
The launch of the new marketplaces, combined with the law's Medicaid expansion, could markedly pare back the uninsured rate starting next year.
Any changes would be reflected in census data released in 2015.
Ron Pollack, a leading liberal healthcare activist, predicted that ObamaCare would have a big impact once its final provisions take effect.
He added that the number of uninsured Americans is a "stark reminder of the important work that lies ahead" as the exchanges open for enrollment in two weeks.