The sale of the paper came after a wrongful termination suit brought by a former CEO, as well as a $3.9-million (€3.27-million) tax bill from the General Department of Taxation (GDT), the Phnom Penh Post reported.
The company's website says that it has previously done work for Prime Minister Hun Sen's government.
Several other members of staff, including managing editor Stuart White, digital director Jodie DeJonge and business editor Brendan O'Byrne, quit in protest at Kimsong's sacking.
On Saturday it was announced that Australian mining businessman Bill Clough had sold the Phnom Penh Post to Sivakumar Ganapathy, a Malaysian investor and executive at public relations firm Asia PR, which includes the Cambodian government among its former clients.
Kay Kimsong, who had worked at the The Post for 10 years, told The Associated Press that the representative of the newspaper's new owner had told him he made a big mistake by allowing the publication on the front page of the article.
The Phnom Penh Put up reported that it had acquired an announcement from its new proprietor saying he was "absolutely dedicated" to upholding the paper's editorial ideas and independence.
Critics said the loss of an independent press would have profound implications in Cambodia, a country that suffered under the brutal rule of the Khmer Rouge.
The main opposition party - Mr Hun Sen's only real rival - was dissolved in a court ruling past year, in a move that rights groups condemned as a death knell for the country's democracy.
Cambodia has elections in July.
"Our article was written in an attempt to maintain the transparency and integrity of our paper", they said.
"I hope this is just a blip in the history of the Post and it continues to be an independent, successful newspaper serving the Cambodian people".
In a statement reportedly from Mr Sivakumar circulating on social media, he described the Post's report as a "disgrace" that "borders on internal sabotage" and said he now had "serious doubts" about the calibre of journalists at the paper.
Since launching in January 2007, the Cambodia-based Southeast Asia Globe has sought to "engage our readers through reports that dig deeper and stories that inspire".
Sivakumar, in the sale's announcements issued separately by himself and Clough, was described as having a background as an experienced journalist and representing a strong investment group from Malaysia.
Asia PR lists Cambodian dictator Hun Sen as former client.
Bill Clough, the former owner and publisher of the Phnom Penh Post, said in a video published previous year that he wanted to maintain the paper's independence.
But Sophal Ear, the analyst, said he saw the language about obeying laws as "a signal to the authorities" about the newspaper's future.
She also carried a message from the newspaper's chief executive Marcus Holmes, who has also resigned. His personal assistant, Krishna Kumar, said Monday in an email that he was unavailable for comment and would be out of the office for about two weeks.