The ferocious typhoon slammed ashore before dawn in Luzon island, a region of flood-prone rice plains and mountain provinces with a history of deadly landslides.
More than five million people were at risk, when winds and gusts equivalent to a Category 4 Atlantic hurricane hit the Philippines.
It caused nearly 150 flights, a third of them international, to be cancelled and halted sea travel.
An adviser to Philippine President Rodrigo Duterte said the victims died mostly in landslides and houses that got pummelled by the storm's fierce winds and rain.
Among the fatalities were an infant and a 2-year-old child who died with their parents after the couple refused to immediately evacuate from their high-risk community in a mountain town.
"They can't decide for themselves where to go," the adviser said of the children, expressing frustration that the tragedy was not prevented.
It is thought the death toll could climb to at least 16 once other casualty reports were verified.
Mangkhut's sustained winds weakened to 105 miles per hour with gusts of up to 161 mph after it blew out to the South China Sea, aiming at Hong Kong and elsewhere in southern China.
About 87,000 people evacuated from high-risk areas of the Philippines. Officials advised them not to return home until the lingering danger had passed.
"It's still a life and death situation," Defense Secretary Delfin Lorenzana said, citing past drownings in swollen rivers in mountain provinces after storms had passed.
Storm warnings remained in effect in 10 northern provinces, including Cagayan, which could still be lashed by devastating winds, forecasters said. Thousands of people in the typhoon's path had been evacuated.
Cagayan's public market was severely damaged, its roof ripped apart and wooden stalls in disarray. Outside a popular shopping mall, debris was scattered everywhere and government workers cleared roads of fallen trees.
Many shops and houses were damaged but most residents remained indoors as occasional gusts sent small pieces of tin sheets and other debris flying dangerously.
The airport terminal was badly damaged, its roof and glass windows shattered by strong winds that also sent chairs, tables and papers flipping about inside.
The typhoon struck at the start of the rice and corn harvesting season in Cagayan, a major agricultural producer, prompting farmers to scramble to save what they could of their crops.
In nearby Fujian province in China, 51,000 people were evacuated from fishing boats and around 11,000 vessels returned to port on Saturday morning.
China's National Meteorological Center issued an alert saying Mangkhut would make landfall somewhere on the coast in Guangdong province on Sunday afternoon or evening.
Ferry services in the Qiongzhou Strait in southern China were halted on Saturday and helicopters and tugboats were dispatched to Guangdong to transfer offshore workers to safety and warn ships about the typhoon, China's official Xinhua News Agency reported.
Mangkhut, the Thai word for mangosteen fruit, is the 15th storm this year to batter the Philippines, which is hit by about 20 a year and is considered one of the world's most disaster-prone countries.
In 2013, Typhoon Haiyan left more than 7,300 people dead or missing, flattened villages, swept ships inland and displaced more than 5 million in the central Philippines.
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