She was one of 60 defendants charged in connection with a failed attempt by an opposition leader to return from exile in 2019.
Mr Biden took up her case with Cambodia’s prime minister Hun Sen, while meeting him on the sidelines of Asean summit. The president “called for the release of activists detained on politically motivated charges, including US-Cambodian dual citizen Seng Theary”, said the White House in a statement following the talks.
Meanwhile, the 51-year-old human rights activist began a hunger strike on Monday as her advocates called for her to be transferred from the prison in Preah Vihear province to Phnom Penh.
According to her lawyers, she has been prevented from attending church services and making regular phone calls, reported Voice of America.
Seng dressed up as Lady Liberty ahead of her sentencing on 14 June. She stood outside the court in a green gown and a crown that had “freedom” written on it and sporting what looked like green glitter on her face and body.
“Free the political prisoners,” she chanted from time to time, raising an imitation torch.
Prime minister Hun Sen’s Cambodia People’s Party subsequently swept the elections that year and won every single seat in a largely symbolic election.
Mr Hun Sen is one of the world’s longest-serving prime ministers, having taken up office in 1985.
While Cambodian authorities had stopped Cambodia National Rescue Party leader Sam Rainsy’s return, they alleged Seng and other defendants were involved in organising it.
Defense lawyer Choung Chou Ngy said the Phnom Penh Municipal Court found Seng and most of the others guilty of conspiracy to commit treason, reported the Associated Press.
While Seng was sentenced to six years in jail, others received sentences of between five and eight years.
Additional reporting from the wires