From A Wiki of Ice and Fire
Warden of the East is the title given to the person responsible for the defense of the eastern coasts of the Seven Kingdoms. Traditionally the title is conferred upon House Arryn, Lords of the Eyrie.
Known Wardens of the East
A Game of Thrones
After the death of Lord Jon Arryn, King Robert Baratheon names Ser Jaime Lannister to the office. This angers the lords of the Vale who take to calling Robert Arryn the True Warden of the East.
A Clash of Kings
To obtain the alliance of the Vale of Arryn and to make peace with Lady Lysa Tully, Tyrion Lannister considers restoring the title to House Arryn.
A Storm of Swords
Lord Tywin Lannister restores the title to House Arryn.
References and Notes