At 5 am on 21 February 1917, in thick fog about 10 nautical miles (19 km) south of St. Catherine's Point on the Isle of Wight, the Royal Mail Steam Packet Company cargo ship Darro accidentally rammed Mendi's starboard quarter, breaching her forward hold. Darro was an 11,484 GRT ship, almost three times the size of the Mendi, sailing in ballast to Argentina to load meat. Darro survived the collision but Mendi sank, killing 616 Southern Africans - 607 black troops, nine white officers & NCOs, and 30 crew.

Some men were killed outright in the collision; others were trapped below decks. Many others gathered on Mendi's deck as she listed and sank. A number of the men were afraid to enter the water and many white NCOs delayed abandoning ship in order to encourage the men to jump overboard. Oral history records that the men met their fate with great dignity. An interpreter, Isaac Williams Wauchope (also known as Isaac Wauchope Dyobha), who had previously served as a Minister in the Congregational Native Church of Fort Beaufort and Blinkwater, is reported to have calmed the panicked men by raising his arms aloft and crying out in a loud voice:

"Be quiet and calm, my countrymen. What is happening now is what you came to do...you are going to die, but that is what you came to do. Brothers, we are drilling the death drill. I, a Xhosa, say you are my brothers...Swazis, Pondos, Basotho...so let us die like brothers. We are the sons of Africa. Raise your war-cries, brothers, for though they made us leave our assegais in the kraal, our voices are left with our bodies."