Beleg was born in Arnor's closing years as a united kingdom. He was given a name means 'great' or 'large', which may be a reference to his stature (although he may simply have been named for the famous Elf of the First Age who shared this name, Beleg Strongbow).
When his grandfather Eärendur died, the old King's sons became divided over the succession. Beleg was fifty years old at this time, and thus it seems likely that he would have been involved to this political dispute (surely taking the part of his father Amlaith, the true heir). Ultimately the North-kingdom was divided into three, with Amlaith becoming King of the reduced realm of Arthedain.
Amlaith ruled over Arthedain for eighty-five years, and on his death Beleg became the new land's second King. During his reign, momentous events took place in Middle-earth. Towards the end of his rule, the Istari or Wizards first appeared. As King of the greatest nation of Men in northern Middle-earth, it seems likely that Beleg was one of the first to encounter them (if this is so, however, the event is not recorded).
More significant still was the secret re-emergence of Sauron, who began to take on a new form in the later years of Beleg's reign, though this was not suspected until many years after Beleg's own time.