A locking mechanism for an armrest assembly for use with a seat of a vehicle, including: an arm portion adapted to be coupled to a pad portion at one end and a structure of the seat at an opposite end and to pivot from an upright use configuration to a collapsed stowed configuration adjacent to the seat; a tube structure adapted to be coupled to the structure of the seat and defining one or more circumferential notches about an exterior surface thereof; an elongate rod coupled to the opposite end of the arm portion and at least partially disposed rotatably within the tube structure; and a tab structure adapted to selectively protrude from the opposite end of the arm portion and engage one of the one or more notches of the tube structure to selectively secure the arm portion in the one or more of the upright use configuration and the collapsed stowed configuration.