Howell was born in around 880, the younger son of Cadell ap Rhodri, himself the son of Rhodri Mawr. In 905, Cadell, having conquered Dyfed, gave it to his son to rule on his behalf. Howell was able to consolidate his position by marrying Elen, whose father had ruled Dyfed until his death. Following his father's death in 909, he acquired a share of Seisyllwg, and on his brother's death in 920, he merged Dyfed and Seisyllwg, creating for himself a new kingdom, which became known as Deheubarth. Following the death of a cousin in 942, he also inherited the principality of Gwynedd, becoming ruler of about three-quarters of present-day Wales.
Howell's reign, uncharacteristically for the time, was a peaceful one, and he achieved an understanding with Athelstan of England. Such was the relationship between the neighbouring countries that Howell was able to mint his own coinage in the English city of Chester. He was the only Welsh ruler ever to produce coinage. His study of the English legal system and his visit to Rome in 928 (on a pilgrimage) combined to enable him to formulate advanced ideas about government. The conference held at Whitland in about 945, was a kind of parliament in which laws were standardised and set down in writing for posterity, much of the work being done by the celebrated clerk, Blegywryd. Following Howell's death in 950, his kingdom was soon split into three and divided between his sons. However, his legacy endured in the form of his enlightened laws, which remained in active use throughout Wales until the conquest.