Reichskonferenz
Mentz was appointed Minister of Lands and Irrigation by Louis Botha in 1915, succeeding Hendrik Schalk Theron. In that capacity he did much to bring about the building of dams, which included the Hartbeespoort Dam and the Darlington Dam in the Sundays River. In 1920 Mentz succeeded General Jan Smuts as Minister of Defence, and in 1921 he attended the Imperial Conference in London. One of the consequences of the Imperial Conference was the abolition of the imperial military command in the Union of South Africa.