In his youth Harwyn raided the Stepstones and visited Free Cities such as Volantis, Tyrosh, Lys, and Braavos. He was a captive in the Basilisk Isles for two years, served as a sellsword in the Disputed Lands, and joined the Second Sons and fought in a number of battles for them.
Harwyn succeeded his father, King Qhorwyn Hoare, even though he was third in line. His eldest brother died of greyscale two years before, and his other brother, Harlan Hoare, died from a fall from a horse. Qhorwyn died six days after his son, and rumours of foul play begun.
Harwyn planned the invasion of the Riverlands, which were, at the time, weakly held by Storm King Arrec Durrandon. The Ironborn landed one hundred longships south of Seagard, and carried them all the way to the Blue Fork.
Ser Samwell Rivers led a small host against him, but was defeated at the Tumblestone. Harwyn then defeated the army of Lady Agnes Blackwood, in part due to the betrayal of Lord Lothar Bracken. Harwyn was, however, impressed by Agnes and offered to make her his salt wife. He executed her after she refused.
Storm King Arrec then met Harwyn in a Battle at Fairmarket. Harwyn was aided by some riverlords, including Lord Lothar Bracken, Theo Charlton, and the lords of houses Goodbrook, Paege, and Vypren. The Durrandons were crushed by the Hoares, and the Storm King's rule over the Riverlands was broken. The rivermen celebrated, as they hoped to regain their independence, but Harwyn took control for himself. Lothar Bracken, who had hoped to be King, rose in rebellion six months later. Harwyn had him starved to death in a crow cage as punishment.