When Albert Ogilvie died suddenly in 1939, 68-year-old
Edmund Dwyer-Gray was elected Labor leader (and thus premier) with the understanding that he would retire after six months in office. Cosgrove was elected as his deputy, narrowly defeating
Thomas D'Alton. He served as
state treasurer until December 1939, when he swapped portfolios with Dwyer-Gray. He appointed himself Minister for Education in 1948, and oversaw "an extensive school-building programme". During the
1955 Labor Party split, Cosgrove was able to prevent the large-scale defections to the
Democratic Labor Party seen in other states. However, the party did split ideologically to some extent, and some individuals (notably
Reg Turnbull and
Brian Harradine) eventually left the party to sit as independents. Cosgrove fell ill in July 1958, and underwent surgery in Melbourne. He retired as premier on 25 August 1958, at the age of 73, and was replaced by his long-serving deputy
Eric Reece, who kept Labor in power until 1969. Cosgrove was premier for 18 years and six months, the longest service in Tasmanian history. Only two other Australian state premiers have served for longer – South Australia's
Thomas Playford IV (26 years) and Queensland's
Joh Bjelke-Petersen (19 years), both of whom used
gerrymandering to hold onto power. In 1959, Cosgrove was appointed
Knight Commander of the Order of St Michael and St George (KCMG), one of the few Labor politicians to accept a knighthood. ==Other activities==