The British annexed the Transvaal Republic in 1876 and Sir Wolseley made Botshabelo his headquarters in the Transvaal. During the First Anglo-Boer War, Merensky was conscripted to the Boer forces as military medic. He took part in the battles of Laing's Nek, Skuinshoogte and Majuba Hill and described the battles viewed from afar from his field hospital. After the end of the war, he was mistrusted by both the British and Boer authorities and decided to move back to Germany with his family. In 1883 he was promoted to Inspector of the Berlin Missionary Society.