A locking mechanism for a terminal position assurance (TPA) device is placed in-line with the TPA itself. The locking mechanism includes a pre-locking head and a final locking head. The final locking head is shorter than and face in an opposite direction from the pre-locking head. The orientation and geometry of the locking mechanism ensures a secure engagement of the TPA with the electrical connector assembly. Further, according to at least one embodiment, at least two pre-lock mechanisms and only a single final-lock mechanism may be provided for the locking mechanism to ensure a secure engagement of the TPA with the connector assembly without a false-positive engagement.