Born in Braunschweig to Eberhard (a wealthy chocolate manufacturer) and Charlotte (née Wittekop), he was famous for undertaking the First German South Sea Expedition at the age of 24.
He fitted out his 300-ton steam yacht Eberhard—purchased from the Prince of Monaco—and sailed to German New Guinea (the Bismarck Archipelago), accompanied by naturalists and anthropologists, including Oskar Heinroth, Paul Kothe, and Georg Duncker.
After reaching Herbertshōhe, he brought on board a former German colonial official, Ludwig Caro, as a secretary.
Mencke's men used firearms to repel the attack, but Caro was speared through and killed along with a few other crew while many members were injured.
[2] A punitive expedition was launched in 1901 by the German colonial forces under (later Admiral) Max von Grapow, which killed about 80 native islanders, including women and children.