[1] He was born in Glengarry County, Canada West, the son of Malcolm McNab and Margaret McCrimmon,[2] and moved to Winnipeg in 1882 with his twin brother before establishing a homestead in Virden, Manitoba.

[1] Years of drought forced him to give up farming and in 1887 he became a grain buyer for Ogilvie Flour Mills.

After selling them, he moved to Saskatoon with his wife and children and established the Dominion Elevator Company.

[2] McNab retired from the legislature when he was appointed to the local government board in 1926 but was forced to resign in 1930 due to allegations of impropriety.

As lieutennt governor, he hosted King George VI and Queen Elizabeth at Government House during their 1939 royal visit to Canada.