In 1927 he was elected as the first Secretary of the United Mine Workers of New Zealand, holding the position until 1935.

[1] McLagan was appointed to the Legislative Council in 1942 by Prime Minister Peter Fraser until he resigned from it in 1946 to contest a seat in the lower house.

[1] Following Labour's defeat at the 1949 general election he was a member of the opposition frontbench under first Fraser and later Walter Nash.

[3] A dour, squarely-built Scot, he was self-educated and well-read, and could write shorthand (which was useful in demolishing the speeches of opponents).

He was grim and aloft; his speeches were cold, logical and often bitter; and some civil servants and others thought he had the best brain in Cabinet.