He remained in close contact with the Lord Lieutenant the Marquess of Ormonde, and was a strong supporter of an alliance between Confederates and Royalists against the English Republicans.
Following the Cromwellian conquest of Ireland, he left for France where he joined the Royalist Court-in-exile and was made Viscount Tara by Charles II.
Since Roman Catholics were not allowed to hold state positions in Ireland, he entered the Spanish service and fought in the Thirty Years' War.
[citation needed] Preston was in the same Irish regiment in the Spanish service as Owen Roe O'Neill, and distinguished himself in the defence of Leuven against the French and Dutch in 1635.
[1] He won widespread praise for his successful siege of Duncannon in 1645, but lost a string of field battles including New Ross (1643) and Dungans Hill (1647).
His Royalism was motivated by his Old English roots and his extreme personal dislike of Owen Roe O'Neill, who led the opposing faction.
He fought with the defeated Royalists during the Cromwellian conquest of Ireland, defending the city of Waterford until hunger and disease forced him to capitulate.
[1] Charles II, who during his exile lodged with the Preston family in Bruges, spoke warmly of the children and their guardian Miss Warren in his later years, but did nothing to repair their ruined fortunes: the younger Thomas in 1670 was said to be penniless.