Johannesburg was not "discovered"—it was founded in 1886 after the discovery of the Witwatersrand Gold Reef, the world's largest gold deposit.
Prospectors (including George Harrison) struck gold on the farm Langlaagte. Within years, a tent camp exploded into a city of 100,000+, drawing migrants globally. By 1900, it was the largest city in Southern Africa.
The gold industry shaped its layout (mine dumps, segregated townships), economy, and labor politics—fueling the Anglo-Boer War and later apartheid's migrant labor system. Today, it remains South Africa's financial capital.