Fifty-eight people were killed in the assault, including 32 media workers, making it the deadliest attack on journalists ever recorded, according to Reporters Without Borders. The accused faced 57 murder counts because one victim's body was never found.
Two brothers named as the masterminds, Andal Ampatuan Jr. and Zaldy Ampatuan, were from a prominent political clan that has backed the central government in battling Muslim separatists in the southern Philippines, a region marred by poverty and insurgent conflict.
Along with other relatives, they face life in prison — which means 40 years in the Philippines — without parole for the killings, which targeted the family of political rival Esmael Mangudadatu. They pleaded not guilty and indicated that they plan to appeal.
The case has captivated the Philippines, where local media have dubbed it the trial of the century. With almost 200 accused, including more than 100 who faced verdicts on Thursday, the proceedings have tested the country's slow-moving and overburdened judicial system.
Outside the court, which sat in a special facility within a prison compound, victims' families emerged grinning and relieved at the conviction of the principal suspects. But they expressed mixed feelings about the overall outcome, given the many acquittals.
"We're all happy with the decision even if not everyone was convicted," said Jergin Dela Cruz Malabanan, 26. She was a teenager when her mother was killed in the massacre, leaving her to raise her siblings and a newborn.
Ahead of the trial, some had pressed for all suspects to face prison sentences.
"If anyone gets out, even if it's just one person, it will hurt. It's not fair," Mary Grace Morales, a 43-year-old mother of three daughters, said at a commemoration last month to mark 10 years since the attack. Her sister and husband were killed in the massacre.
In 2009, Mangudadatu's family and lawyers, accompanied by a media entourage, had been on the way to lodge his candidacy in a gubernatorial run, challenging the Ampatuan family in Maguindanao province. They were ambushed, gunned down and buried in a mass grave surrounded by corn and grass fields.
The massacre cast light on a culture of impunity and the power wielded by political dynasties in the Philippines — as well as dire working conditions for journalists in the country, which ranks consistently among the most dangerous for media workers.
Family patriarch Andal Ampatuan Sr., another suspected architect of the massacre, died in 2015.
Since the trial began, three witnesses have been killed. Families of the victims have reported continued harassment and threats.
Human Rights Watch said the verdict would help as they work to gain greater accountability for rights abuses in the Philippines.
"More broadly, this verdict should prompt the country's political leaders to finally act to end state support for 'private armies' and militias that promote the political warlordism that gave rise to the Ampatuans," said Phil Robertson, Human Rights Watch's deputy Asia director.
Ahead of the judgment, the rights group condemned authorities' inability to capture the dozens at large, noting that some of the suspects were police and military men who worked with the Ampatuans.
President Rodrigo Duterte, a populist leader who has promised to resolve the case, welcomed the court's ruling.
"This savage affront to human rights should never have a duplication in this country's history," presidential spokesman Salvador Panelo said Thursday. Panelo was previously legal counsel to the Ampatuans.