His family moved to Dunedin soon after his birth and he received his education at Otago Boys' High School, which he attended from 1903 to 1905.

[1][2][3] Mazengarb was joined by Ernst Peterson Hay and Robert Macalister and their practice soon rose to one of the largest in the capital city.

[6][7] In the 1953 Coronation Honours, Mazengarb was appointed a Commander of the Order of the British Empire, for charitable and public services, especially in the field of law.

[8] A well-known public appointment was in 1954, by the National government of the time, to chair the Special Committee on Moral Delinquency in Children and Adolescents, otherwise better known as the Mazengarb Report.

On 6 April 1920 at St John's Church in Invercargill, Mazengarb married Margaret Isabel Campbell.