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.
The changes in Montgomery and Prince George's have an outsized effect on how the state looks. For example, the state lost about 129,000 non-Hispanic white residents over the decade. Almost all - 106,000 - left Montgomery and Prince George's. The statewide loss represented a 4 percent drop, much steeper than the 1 percent drop experienced during the 1990s.
The two counties gained about 137,000 Hispanic residents during that period. In Prince George's, the Hispanic population more than doubled in size, from 7 percent of the county to 15 percent As their ranks swelled, they surpassed a white population that has dropped greatly, from being 24 percent of the county in the last census to 15 percent today.
"Those are incredible numbers," said Prince George's County Council member Will Campos (D-Hyattsville), a native of El Salvador who came to the United States in 1983 when he was in the third grade.
Then, there were only one or two other Latinos in his class, he said. Now, the schools in his district are filled with Hispanic students, said Campos, who was the first Hispanic elected to the County Council.
As its white population slipped below 50 percent, Montgomery was catapulted into the top tier of majority-minority counties that are wealthy.
"What we're seeing is a reinvigoration of the suburbs with Hispanics, Asians and blacks," said William Frey, a demographer with the Brookings Institution. "It makes Washington more ethnic, more globalized, while the white suburban image is pushing further outward."
Frey noted that whites are dwindling not only in the inner suburbs abutting the District but also in neighboring counties such as Anne Arundel and Howard.
"It makes them look like they're becoming more like the D.C. region is demographically," he said.
Eventually, the region's delegation to Annapolis is expected to look more like the region, too.
Although Montgomery has some Latino and Asian state delegates, all eight of its state senators are white.
"The delegation is clearly whiter than the constituency at this point. That will change and probably should change at some point, depending on how the districts are redrawn," said Tom Schaller, a professor of political science at the University of Maryland, Baltimore County.
Staff writers Aaron Davis, Michael Laris and Miranda Spivack contributed to this report.