After the defeat of the ALP at the 1968 election, Corcoran became Deputy Opposition Leader under Don Dunstan. When Labor won the 1970 election, he became Deputy Premier under Dunstan and assumed many ministerial portfolios including Marine and Harbours and Public Works. He became Premier on February 15, 1979 after Dunstan's shock resignation and assumed the positions of Treasurer and Minister for Ethnic Affairs. Spurred by positive opinion polls, he called an early election, but after a disastrous campaign, the election was lost with an 11% swing to the Liberal Party.
Corcoran retired from politics in 1982 and died from cancer in 2004.
Corcoran founded the Gallery for the purpose of "encouraging American genius" in the arts and built the first Gallery to house his collection of paintings and statuary.
Corcoran then gave, in addition to the original building and grounds and his personal art collection, an endowment of $900,000 for the perpetual establishment and maintenance of the Gallery.
Not only is the Corcoran an architectural achievement in the Beaux Arts tradition, but its continual dedication to art is a contribution to the cultural heritage of the nation's capital.