Dr. Quát then served briefly as an interim prime minister until Bảo Đại appointed Ngô Đình Diệm to the position.

In April 1960, Quát signed the Caravelle Manifesto, a list of grievances and demands specifically critical of Diệm, and was promptly jailed by the GVN.

After Diệm's assassination in October 1963, Quát was appointed Foreign Minister by Major General Nguyễn Khánh, one of the principal participants in the bloody coup.

[4] On 16 February 1965, the Armed Forces Council, a group of South Vietnamese military officers who took over when General Khánh deposed Hương and the HNC, secured Quát's appointment to prime minister in order to foil a power grab by the junta chief Khánh, who intended to install the economist Nguyễn Xuân Oánh as his puppet in the Prime Minister post.

He remained in politics until 1975 by working with the Asia Anti-Communist League (Liên Minh Á Châu Chống Cộng) as chairman of its Vietnamese office.