[1][2][3] During the Second Boer War, Mentz fought under General Ben Viljoen in Natal, being involved in the siege of Ladysmith and the battles of Colenso and Spion Kop.

[4][3] After the war Mentz settled in Pietersburg, where he practised law and when the Transvaal Colony obtained responsible government in 1906 he was elected to the Legislative Assembly as a supporter of General Louis Botha.

He was appointed military governor of Windhoek by Louis Botha, whereafter his brigade joined the final advance on the north, which culminated in the surrender of the Germans.

[1] Mentz was appointed Minister of Lands and Irrigation by Louis Botha in 1915, succeeding Hendrik Schalk Theron.

In 1920 Mentz succeeded General Jan Smuts as Minister of Defence, and in 1921 he attended the Imperial Conference in London.