In December 1970 Drees was approached by Jan van Stuijvenberg, the chairman of the newly founded Democratic Socialists '70 (DS'70), to seek the leadership for the 1971 general election. Drees accepted and was unopposed in his candidacy and was elected as leader and became the lead candidate of the Democratic Socialists '70 for the election on 8 January 1971; he resigned as Treasurer-General that same day. After the election the Democratic Socialists '70 entered the House of Representatives with 8 seats. Drees became the parliamentary leader of the Democratic Socialists '70 in the House of Representatives, taking office on 11 May 1971. Following the cabinet formation of 1971, Drees was appointed as Minister of Transport and Water Management in the Biesheuvel I cabinet, taking office on 6 July 1971. The cabinet fell just one year later on 19 July 1972 after DS'70 retracted its support following their dissatisfaction with the proposed budget memorandum to further reduce the deficit. The DS'70 cabinet members resigned on 21 July 1972. For the 1972 general election Drees again served as lead candidate. The Democratic Socialists '70 suffered a small loss, retaining 6 seats in the House of Representatives. Drees returned to the House of Representatives and resumed his position as his party's parliamentary leader, taking office on 5 September 1972. For the 1977 general election Drees once more served as lead candidate. DS'70 suffered a big loss, losing 5 seats and keeping only 1 seat in the House of Representatives. Drees took responsibility for the defeat and subsequently announced he was stepping down as party leader, parliamentary leader and member of the House of Representative on 20 August 1977.
On 3 February 1947, he married Anna Erica Gescher (born 26 October 1922). They had five children; four girls and one son Willem B. Drees, their third child, became a philosopher. Anna Erica Drees-Gescher died on 12 May 1988 at the age of 65. Willem Drees Jr. died on 5 September 1998 at the age of 75. Relatively young compared to his parents; his father Willem Drees died at the age of 101 and his mother Catharina Hent died at the age of 85.