He was King during a time of crisis. In the beginning of his reign there already were frequent skirmishes with the Dunlendings who operated from captured Isengard.
Later in his reign a man named Freca appears, a Dunlending with Rohirric blood. Freca claimed to be a descendant of King Fréawine of Rohan. Freca engaged in a big scheme to gain to throne of Rohan for his kin. He planned to complete the scheme by letting his son, Wulf, marry the daughter of Helm. But the King refused and thus there came open war with the Dunlendings.
Under the lead of Wulf the Dunlendings overran Rohan. But Helm withdrew in the Hornburg at Helm's Deep (named in his honor). There he endured a long siege.
Then he sortied and blowed his horn, and where the horn sounded fear was installed upon the Dunlendings . However Helm Hammerhand and his sons Haleth and Háma were slain. The Dunlendings still believe that the wraith of Helm Hammerhand wanders through Rohan.
He was succeeded by his nephew Fréaláf.