Daniel had two elder sisters and two younger brothers including Commander (Kommandant) Stephanus Petrus Erasmus (nicknamed Swart Lawaai, around 1850 – 1921).

The widower then married Sibella ("Beppie") Margaretha Aletta Schabort (Vijgenboom, Western Cape, 13 March 1852 – Grootfontein, Pretoria, 13 October 1912) on 25 April 1876 in Heidelberg.

Daniel Erasmus took part in the Battle of Bronkhorstspruit of 20 December 1880 during the First Boer War (1880-1881) and besieged the British garrison in Pretoria, blocking three outbreaks.

At a general Boer council of war on 11 October 1899 at Zandspruit/Sandspruit, just over the border of Natal Colony east of Newcastle and south of Utrecht, Erasmus was nominated Assistent Kommandant Generaal, second in command to Transvaal Commander-in-chief Piet Joubert.

Erasmus was transferred to a prisoner of war camp at Saint Helena, where he remained for five months until his release as a consequence of the peace Treaty of Vereeniging signed on 31 May 1902.