Pieter Mattheus Kruger Le Roux was born on 11 November 1904 at Doornhoek, a farm near De Rust in the vicinity of Oudtshoorn in what was then the Cape Colony.

His parents were Gert Cornelis Le Roux and his wife Margaretha.

He attended Outeniqua High School in George, then the University of Stellenbosch.

He was appointed to the Cabinet as Minister of Water Affairs and Agricultural Technical Services in 1958, shortly before the end of J. G. Strijdom's premiership, and retained this role under Strijdom's successor Hendrik Verwoerd until 1966, when B. J. Vorster became Prime Minister following Verwoerd's assassination.

Vorster made Le Roux Minister of the Interior, a post he retained until 1968.