Manifold's exit from BP's top post is unexpected and comes ahead of a planned restructuring in the wake of several years of profitability problems.
BP's shares plunged by over 6 percent on the London Stock Exchange after the announcement.
Manifold took office last year and played a crucial role in the dismissal of former BP CEO Murray Auchincloss and the recruitment of Meg O'Neill as his replacement.
The board has appointed board member Ian Tyler as acting chairman. Tyler says in a statement that he believes in the turnaround plans and in Meg O'Neill as CEO.
