Obama has designated 13 such monuments during his presidency, protecting a total of 260 million acres of both land and water, and sparking outrage from Republicans in Congress.
"Within these hills lies a millennia of history. Just as this region teaches us about our past it offers us a window into the future," Obama said at an event in Los Angeles County.
"I can think of no better way to honor our past and protect our future then by preserving the San Gabriel Mountains," he added.
Obama has designated nearly three times more land and water than any other president since Congress first passed the Antiquities Act in 1906, according to the White House.
Obama's predecessor, former President George W. Bush, comes in at a close second though, having designated at least 200 million acres of marine reserve near Hawaii at the end of his adminstration.
The designation for the San Gabriel Mountains comes after a decades-long push by Hispanic and environmental groups for the federal government to name it a national monument.
The new protections will preserve the thousands of acres, which provide 30 percent of the drinking water for some 15 million people living in the surrounding region, and 70 percent of L.A. County's open space, Obama said on Friday.
A lack of funding has plagued the mountains, which include the Angeles National Forest. The forest is one of the busiest in the country, with more than 3.5 million visitors each year.
The monument declaration will help fund and improve trail signs, restrooms and trash facilities, Obama said.
"It's not enough to have this awesome natural wonder, you need to be able to access it," he added.
On top of the designation, Obama announced that a number of philanthropies will commit to help kick-start public involvement and restoration projects in L.A. County, and the new monument.
The National Forest Foundation is pledging $3 million for a new San Gabriel Mountains National Monument Fund to aid in community response and restoration. Another donation from the California Endowment and Resources Legacy Fund will help support recreation and habitat improvement projects.
The Hispanic Access Foundation praised Obama for the new monument.
"The Latino community recognizes how important quality drinking water, clean air, and accessible public lands are to the well-being of southern California — particularly in an area that has limited green space,” said Maite Arce, CEO of Hispanic Access Foundation. “President Obama’s action will help protect these resources for future generations.”
Republicans were quick to blast Obama for his latest designation, coming less than one month after his biggest move yet to protect the oceans by declaring a large swath of the Pacific off-limits to fishing and energy exploration.
“Once again this Administration is taking unilateral action without Congressional or public input by naming the San Gabriel Mountains a National Monument," said House Majority Leader Kevin McCarthy (R-Calif.).
"This action restricts access to public lands and would put nearly half of the Angeles National Forest under lock and key. I strongly support multiple use of our national forests and other public lands, but this decision severely limits usage," McCarthy added.
Despite the pushback, Obama insisted on Friday he plans to designate more land and water before the end of his presidency.
"I've preserved 3 million acres of public lands for future generations and I'm not finished," Obama said.
"We are looking at additional opportunities to preserve federal lands and waters and I will continue to do so especially where communities are speaking up," he added.