Hart served in the Second Boer War from late 1899, first in command of the Northern Company of Mounted Infantry, being present during all the operations about Naawpoort and Colesberg with French's Cavalry Division (for which he was mentioned in despatches). He commanded the company in the relief of Kimberley (15 February 1900) and in the advance on Bloemfontein, including the battle of Paardeberg during which he was wounded, the engagements at Driefontein, Poplar Grove, and Klip Drift, and in the subsequent occupation of Bloemfontein and operations east of that city about Thabanchu. In summer 1900 he served with Hamilton's Force in command of the Northern Company of Mounted Infantry and afterwards as Staff Officer of Legge's Corps of Mounted Infantry during the advance to Pretoria and the following capture of Johannesburg and Pretoria. He took part in the Battle of Diamond Hill (June 1900), and as staff officer of Mounted Infantry in the march to Betlehem and Vredefort, following which he served as brigade major of the Irish brigade with Lord Kitchener's column in pursuit of Boer general Christiaan de Wet.