De Castro's departure comes just weeks before new America Online chief executive Jon Miller was due to pitch his turnaround plan for the unit to the company's board and analysts.
De Castro was appointed interactive services president by former AOL time Warner chief operating officer Robert Pittman, who himself resigned in May following a few short months at as the caretaker chief of the online unit.
AOL has been hit by a slowdown in subscriber growth and a drop-off in advertising revenue, as well as a US government probe into its accounting.
