Born in the Cape Colony but raised in the South African Republic, Cronjé had a distinctive appearance, being short with a black beard and was reputed to have considerable personal courage.

[5] After Lord Methuen attempted to relieve the siege of Kimberley, Cronjé fought the Battle of Modder River on 28 November 1899, where the British won a Pyrrhic victory over the Boers.

Cronjé's novel tactics at the Modder River, where his infantry were positioned at the base of the hills instead of at the tops—to increase the effectiveness of their rifles' flat trajectories—earned him a place in military history.

The commanding officer of the 3rd Battalion, the Grenadier Guards, Lieutenant Colonel Eyre Crabbe, was surprised to find that Cronjé had been accompanied on the campaign by his wife.

Dubbed a "circus general" by the South African press, he failed to return home at the time, instead joining a show on Coney Island, Brooklyn.