Edgar was made King of Mercia and Northumbria in 957 and succeed to the throne of Wessex at his brother, Eadwig's, death in 959. With this, Edgar was King of Mercia, Northumbria and Wessex (the three most powerful kingdoms in England at that time), simultaneously and could be considered the first ruler of a United England. Some of his predecessors were Kings of All England by virtue of being King of Wessex and, at the same time, enjoying a temporary military ascendancy over the other kingdoms.
He was formally crowned in 973 and received the ceremonial submission of all the other kings in Britain. He wisely recalled (St.) Dunstan from exile and made him Archbishop of Canterbury and his closest personal advisor. His reign was prosperous and peaceful and he is generally credited with the revival of the English church.
All you need to know about King Edgar.