Published: 2017-02-27 12:10:01 BdST
It was the first Oscar for Affleck, 41, the younger brother of "Argo" director and actor Ben Affleck.
Affleck's understated performance as a taciturn working man who unexpectedly has to look after his teenage nephew beat challenges from Denzel Washington and Ryan Gosling to win the Academy Award.
Affleck swept awards in the run-up to the Oscars, taking home a Golden Globe and numerous trophies from film critics groups.
The wins came despite 2010 sexual harassment allegations that resurfaced as Affleck's Oscar prospects gathered steam. Two civil lawsuits alleging unwanted advances were filed by female crew members on another movie, and were settled out of court for undisclosed sums. Affleck's lawyer at the time denied the allegations.
Ironically, the "Manchester by the Sea" role was first intended for Matt Damon, a Boston childhood friend of Casey and Ben Affleck. Damon had to step down because of other commitments.
An emotional Affleck thanked fellow nominee Washington, whom he credited with being one of the first people to teach him to act, as well as Damon and the film's screenwriter and director, Kenneth Lonergan.
"I'm really proud to be part of this community," Affleck said. "I'm just dumbfounded that I'm included."
Affleck adopts a much lower public profile than his brother Ben and appears uncomfortable in the spotlight. He was first nominated for an Oscar in 2008 for his lead role in "The Assassination of Jesse James by the Coward Robert Ford."
While Ben Affleck has opted for big movies like "Batman v. Superman: Dawn of Justice," Casey Affleck has focused his career on smaller, independent ventures with strong character-driven plots such as "Gone Baby Gone."
In 2010, he wrote and directed Joaquin Phoenix in "I'm Still Here," a mockumentary about Phoenix's supposed transition from acting to becoming a rap musician.
Affleck was raised in the Boston area, started work as a child in TV shows, moved to Los Angeles after high school, and then dropped out of acting for a while to attend Columbia University in New York.
Two years ago he quietly separated from his wife, actress Summer Phoenix, after a nine-year marriage that produced two sons.