Thomas was educated at Delany's school in Dublin, and Trinity College, Dublin, where he matriculated on 22 November 1685 and left without taking a degree. Prior to the outbreak of the 1688 wars he is likely to have left Ireland for London with his father. He returned to Ireland in the army of King William III, as a lieutenant in Lord Lovelace's regiment of foot, and served at the Siege of Limerick. This may have been followed by a brief spell in the Irish engineers from 1691. In any case, de Burgh was commissioned as a Captain in 1692 in the Royal Regiment of Foot. In this capacity he served in the Low Countries at the battles of Steenkerke (1692) and Landen (1693), and as engineer at the siege of Namur (1695). During this time, he absorbed the ideas of the Dutch engineer Menno van Coehoorn (1641–1740). In 1697 he became Third Engineer on the Irish establishment.