He served in the Legislative Assembly of Manitoba from 1943 to 1959, and was a cabinet minister in the governments of Stuart Garson and Douglas Campbell.
In 1943, persuaded by his childhood friend, Douglas Lloyd Campbell, to pursue provincial politics, he was first elected to the Manitoba legislature in a by-election held on November 18, 1943, replacing Toby Sexsmith.
When Douglas Campbell replaced Garson as Premier of Manitoba on December 14, 1948, he named Greenlay as his Minister of Labour.
[1] During this time, he was instrumental in the leadership to create a recreational area for all Manitobans to enjoy, the Whiteshell Provincial Park.
The Liberal-Progressives were defeated by the Progressive Conservatives in the 1958 provincial election, although Greenlay was re-elected in Portage la Prairie[1] with an increased majority.
Manitoba CCF leader Lloyd Stinson once described Greenlay as "a mild, quiet little man, not lacking in ability but essentially cautious and stubborn".
During his entire life, Greenlay maintained a cottage at Delta Beach, where he enjoyed fishing, hunting, as well as entertaining many friends and family.