The US population hit the historic milestone of 300 million on Tuesday at 7:46 am (1146 GMT), the Census Bureau announced.
The date and time for the arrival of the 300 millionth US resident was based on a calculation by the Census Bureau that takes into account birth rates, death rates and rates of international migration.
With that formula in mind, it was determined that the 300 millionth resident clocked in at 7:46 am (1146 GMT) and was either a new-born, an immigrant coming across the border or someone flying into the United States.
Unlike the pomp and circumstance that greeted the 200-million mark in 1967, federal officials did not organise any major celebrations for Tuesday's landmark, which comes amid a raging debate on immigration and concerns over the potential environmental impact.
Experts say that one reason the population has grown so much in the past 39 years is because of the large number of immigrants, many of them illegal, coming into the country, especially in the past 15 years.
"About half of the growth in the US population is due to Hispanics, and well over half the growth is due to immigrants and their children," said William Frey, a demographer at the Brookings Institution in Washington.
He said the growth represents a return to the United States' immigrant past and a return of its image as a "melting pot".
"This is important because it's a symbol of where America is going in the new century," he said.
Global Insight chief economist Nariman Behravesh said the fact that the US population is growing faster than that of other developed countries was very good news.
"It will allow us to better deal with the budgetary pressures (Social Security, Medicare, etc.) related to our ageing population," he said. "Given our large land mass, we have had no problem absorbing all these new people.
"In fact, our population could double and we would still have a lower population density than Japan and most European countries."
Some, however, are expressing concern about the environmental impact of the growing US population, which is expected to reach 400 million by mid-century and 600 million by the end of the 21st century.
"In some ways, you have to appreciate the kind of growth and prosperity in your country, but on the other hand, I see what kind of implications this has for the natural environment," said Vicky Markham, director of the Center for Environment and Population, based in Connecticut.
Markham pointed out that although the United States currently represents about five percent of the world population, its citizens are wreaking havoc on the environment at a rate surpassing any other country.
Americans, for example, produce five pounds of trash daily, five times the average in developing countries, and the country is losing 3,000 acres of farmland every single day, mainly for development purposes.
"The US is the only industrialised nation in the world with significant population growth and that, combined with our rate of resource consumption, results in America having the largest environmental impact in the world," Markham said.