The original Mockingbird's identity was unrevealed in the original series, but later identified as August Durant. After the death of almost the entire original Secret Six team (including Durant) his teammate Carlo DiRienzi succeeded him in the role, to lead a new team. Upon DiRienzi's death, his son Rafael DiRienzi, thought to be his successor in the role, disappeared amid belief he would succeed his father, and has was never seen again.
The first Mockingbird of the villainous Secret Six was revealed to be Lex Luthor. After he gave up on the team, they worked independently until a new Mockingbird appeared giving them missions. This Mockingbird was revealed to be Amanda Waller. In the New 52, the Mockingbird was revealed to be the Riddler.