Lieftinck applied at the Columbia University in New York City in April 1929 for a postgraduate education and obtained a Master of Financial Economics degree in December 1930 and later returned to the Utrecht University where worked as a researcher and got a doctorate as a Doctor of Philosophy in Public economics on 10 December 1931.

Lieftinck worked as a professor of Public economics at the Erasmus University Rotterdam from 11 October 1934 until 30 March 1940.

Lieftinck also served in the military reserve force of the Royal Netherlands Army and was mobilized in March 1940.

On 10 May 1940 Nazi Germany invaded the Netherlands and the government fled to London to escape the German occupation.

Lieftinck fought in the Battle of the Netherlands and the Battle of France and was captured following the capitulation and detained in Kamp Sint-Michielsgestel from October 1940 until he was transferred to Haren in November 1940 and was transferred to Buchenwald concentration camp in April 1941 and was released in January 1943.

Following the end of World War II Queen Wilhelmina ordered the formation of a cabinet of national unity to reorganize the state and make preparations for new elections with Lieftinck appointed as Minister of Finance in the Cabinet Schermerhorn–Drees, taking office on 25 June 1945.

In April 1948 Lieftinck announced that he wouldn't stand for the election of 1948 but wanted run for the Senate.

After the election of 1948 Lieftinck continued as Minister of Finance in the Cabinet Drees–Van Schaik, taking office on 7 August 1948.

Before the second world war, he supported the left wing of the Protestant Christian Democrats (CHU).

He worked with other politicians on ideas of a new political constellation after the second world war in the breakthrough committee.

In the Netherlands, exports of money, goods and services to nazi Germany were credited to a Reichsmark account in Berlin that was, of course, never paid.

A great degree of leveling of wealth had occurred and the economy had been put on a new and sound footing; both points had been high on the wish list of the Beekvliet reformers.

After retirement in 1976, he remained active as an advisor to successive Dutch and Suriname governments on matters of economic and financial reform.