Under the colonial French-imposed constitution, any member of the Norodom or Sisowath branches of the family could be selected as king, and Sirik Matak was therefore one of the possible contenders to the Cambodian throne.

Sihanouk later accused Sirik Matak of harbouring a deep resentment against him, stating that he "hated me from childhood days because he thought his uncle, Prince Sisowath Monireth, should have been placed on the throne instead of myself.

Despite the incorporation of much of the right-wing opposition into the Sangkum, Sirik Matak remained an implacable opponent of Sihanouk, and especially of the latter's toleration of North Vietnamese activity within Cambodia's borders.

After being appointed as Lon Nol's deputy, he proceeded to organise a series of economic denationalisation and deregulation measures in opposition to Sihanouk's previous policy of state control of import and export, banking, and production of pharmaceuticals and alcohol.

[15] Sirik Matak also had relatively little personal support within the Cambodian political establishment; his power was gradually undermined by the Prime Minister's brother, Lon Non, and he resigned in 1972 after the latter had organised a series of demonstrations against him.

[16] Despite pressure from the United States, who were strong supporters of Sirik Matak, Lon Nol kept him under effective house arrest, and he became an increasingly vocal critic of the Khmer Republic regime.

On 1 April 1975, President Lon Nol resigned and fled the country into exile in Hawaii; the Khmer Rouge had published a 'death list' with his name at the top, and their forces had now surrounded the capital.

On 12 April 1975, United States Ambassador to Cambodia John Gunther Dean offered high officials of the Khmer Republic political asylum in the United States, but Sirik Matak, Long Boret and Lon Non, along with other members of Lon Nol's cabinet, declined – despite the names of Boret and Sirik Matak being published by the Khmer Rouge in a list of "Seven Traitors" marked for execution.

Shortly after the surrender to the Khmer Rouge was announced, Sirik Matak sought refuge at the Hotel Le Phnom, where the International Red Cross was attempting to create a safe zone.

[22] François Bizot reported that Sirik Matak sought political asylum at the French Embassy and that the Khmer Rouge threatened to come into the compound and remove certain individuals by force if they did not go voluntarily.

The exact details of his death are unclear, but Sihanouk received confirmation that Sirik Matak, along with Long Boret, had been summarily executed by firing squad at the Phnom Penh Cercle Sportif on 21 April; other reports state he was beheaded.