A mixing container having an outer vessel with an open mouth and an inner vessel also with an open mouth separately stores two liquids for automatic mixing when the container is opened. Upon assembly, an interior seal between the open mouth of the inner vessel and the bottom of the outer vessel is effected through placement of a closure in the open mouth of the outer vessel. The inner vessel is received by a socket mounted in the closure which provides a resilient bias on the inner vessel to force the open mouth of the inner vessel against the bottom. A method for filling the mixing container includes a filling of the inner vessel, placement of the outer vessel over the inner vessel, immersing at least the open mouth of the outer vessel into a second liquid and evacuating the outer vessel. The mixing container may then be closed to resiliently bias the open mouth of the inner vessel against the bottom of the outer vessel to effect the interior seal.