A medicament delivery device includes a distal housing part; a proximal housing part for a container; a drive mechanism in the distal housing part that includes a plunger rod; and an auto mixing mechanism. The housing parts are movable from a first position, in which the housing parts are partially extended relative to each other, to a second position, in which the housing parts are partially retracted relative to each other. The auto mixing mechanism is configured to hold the housing parts in the first position and to lock the housing parts in the second position, and includes a mixing force member operably arranged between the housing parts such that manual activation of the auto mixing mechanism enables the mixing force member to cause the housing parts to move from the first position to the second position. The plunger rod acts to mixing a medicament in the container.