Cameron said the total raised -- nearly $6 billion in 2016, and a further $5 billion until 2020 -- exceeded the $9 billion being sought from the assembled donor countries.
"It means millions of people will now receive lifesaving food, medical care and shelter in Syria and beyond."
Britain's Foreign Office said it was the largest amount raised in a day for a humanitarian crisis.
Aleppo offensive worsens crisis, Turkey says
But Turkish Prime Minister Ahmet Davutoglu warned delegates at the conference that a Russian bombing campaign in Syria's Aleppo province was driving a new exodus of refugees toward the Turkish border.
Syria's opposition cited the intensive air campaign among its reasons for calling a halt to critical peace talks Wednesday in Geneva, Switzerland
And on Thursday, Davutoglu told delegates that the offensive was worsening Syria's humanitarian crisis.
"Let's be clear, Syrians are fleeing because they are being bombed by the regime and their supporters," he said, as a renewed war of words escalated between Ankara and Moscow, who back opposite sides in Syria's civil war.
Russia is one of Syrian President Bashar al-Assad's main backers, while Turkey supports Syria's opposition.
A Thursday air raid on rebel-held neighborhoods in Aleppo, which Russian aircraft are believed to have carried out, killed at least 21 people, according the Syrian Observatory for Human Rights, a UK-based monitoring group.
That operation followed an announcement by Syrian state media Wednesday that government forces, supported by Russian planes, had broken a 3½-year rebel siege of a pair of pro-regime Shiite villages north of Aleppo, and severed a key rebel supply route linking Aleppo to the Turkish border.
Turkey, Russia trade words
As tensions between Ankara and Moscow simmer -- particularly since Turkey's downing of a Russian military aircraft in November
-- a Russian military spokesman expressed suspicion Thursday that Turkey was preparing a major military incursion into Syria, according to Russian state news agency RIA Novosti.
"We have a serious basis to suspect intensive preparation by Turkey to a military invasion into the territory of the north of the Syrian Arab Republic," the agency reported Maj. Gen. Igor Konashenkov, a spokesman for the Russian Ministry of Defense, as saying.
A source within the Turkish Prime Minister's office denied the report.
"Russians are trying to hide their crimes in Syria," the source told CNN.
"They are diverting attention from their attacks on civilians as a country already invading Syria. Turkey has all the rights to take any measures to protect its own security."
Jordan: 'We have reached our limit'
Seventy countries and international organizations gathered for the London conference.
It aimed to raise more than $7 billion requested by the United Nations for Syria, and about $1.3 billion requested by countries in the region -- Jordan, Lebanon and Turkey -- which are shouldering the brunt of the humanitarian burden caused by crisis, housing about 4.6 million refugees.
"Today one of every five people living in our kingdom is a Syrian refugee. It is as if the UK had absorbed the entire population of Belgium," Jordanian King Abdullah told attendees.
"We have reached our limit. Our country will continue to do what we can do to help those in need, but it cannot be at the expense of our own people's welfare."
According to the United Nations, nearly nine in 10 of the 1.2 million Syrian refugees in Jordan live below the poverty line.
But as U.S. Secretary of State John Kerry observed at the conference: "The challenge is not to just write checks.
"To stop the flow of refugees," he said, "we have to end the war."
Peace talks temporarily suspended
Those efforts ground to a temporary halt Wednesday when U.N. special envoy Staffan de Mistura announced "a pause" in discussions in Geneva, saying they should restart February 25.
The talks, aimed at creating a nationwide ceasefire in Syria, broke up amid opposition anger over the brutal Russian air campaign over Aleppo province.
The United States, France and Turkey have criticized the airstrikes, but Russian Foreign Minister Sergey Lavrov vowed Wednesday to continue the strikes "until we truly win over terrorist groups."
Kerry said that he had spoken with Lavrov on Thursday, adding both parties "agreed that we need to discuss how to implement the ceasefire."
Russian airstrikes have killed nearly 1,400 Syrian civilians since beginning in September, the Syrian Observatory for Human Rights said last week.
The Russian intervention in the Syrian conflict has helped the Assad regime, which had suffered major setbacks before Moscow entered the fray.
Elsewhere in Syria, the International Committee of the Red Cross announced that food and relief aid had been delivered Wednesday to more than 12,000 in the besieged rebel-held town of Moadamiyeh near Damascus.
The ICRC said the town of 50,000 needs more aid, and it hoped to deliver it in the coming days.