McNair was appointed Attorney-General in Premier Dysart's cabinet on July 16, 1935. He held this position for the next seventeen years, until his retirement from politics in 1952. In the Legislative Chambers, McNair distinguished himself as a gifted speaker and was widely regarded as the most able member of Dysart's government. Dysart frequently suffered from ill-health and during his many absences from the Legislature McNair served as Acting premier. Although defeated in York County at the general election of November 20, 1939, a by-election was created for McNair in Victoria County and on January 20, 1940 he was re-elected.