Barely 49 percent of Montgomery's 972,000 residents are non-Hispanic whites, down from almost 60 percent in 2000 and 72 percent a decade before that. Hispanics rose by two-thirds and make up about 17 percent of the county's population.
The census figures surprised some residents but reinforced what's readily evident.
"Wow. That's incredible," said Montgomery County Council member Nancy Navarro (D-Eastern County), who immigrated to the United States from Venezuela when she was in the fifth grade. "This changes the image of Montgomery County as just a wealthy, mostly Caucasian county.
"A lot of us know intuitively what happened, but it's different when you have it in real numbers," she added. "We have to recognize what the Montgomery County of today looks like."
The major demographic shifts mirror changes underway throughout much of the region and state.
As recently as 1990, seven out of 10 Maryland residents were white. Now, they are barely a majority, at 55 percent of the population. For the first time, a majority of the state's children younger than 18 are minorities, an important harbinger of growth as those children come of age.
Although the rate of growth in Montgomery and Prince George's slowed over the past decade, it continued to outpace that of the Baltimore suburbs. As a result, the two counties remain home to one in three of the state's residents, making the voter-rich area vital at election time. In contrast, the state's traditional political powerhouse - Baltimore City - continues to lose ground, although at slower clip than during the 1990s.
Baltimore was the only jurisdiction in Maryland to lose population, and it is likely to drop from six seats to five seats in the state Senate when districts are redrawn using the new census statistics. Montgomery will maintain eight seats, and Prince George's will keep at least seven, analysts said.
The tilt could help politicians from Washington's Maryland suburbs more easily ascend to statewide office.
The three Democrats most often rumored to run for governor in 2014 - Lt. Gov. Anthony G. Brown, Attorney General Douglas F. Gansler and Comptroller Peter Franchot - are from the Washington region.
Montgomery and Prince George's were not the only places to undergo growth and transformation over the decade.
Some of the state's biggest growth was in Southern Maryland, in Charles and St. Mary's counties, where the population rose more than 20 percent.
Charles joined Montgomery and Prince George's as a majority-minority county during a decade of dizzying change. The percentage of whites in Charles plunged from 67 to 48 percent in just 10 years - a shift generated by the arrival of thousands of blacks and, to a lesser extent, Hispanics and Asians. The number of blacks nearly doubled as a wave of African Americans arrived from Prince George's and the District.