Mixing devices used with a syringe or syringe system and related systems for mixing dissimilar components contained therein. The devices and systems include a hollow elongate handle having a proximal end, a distal delivery end, and a delivery passageway therethrough. A mixing member is disposed at or near the proximal end of the hollow elongate handle, which mixing member and a portion of the handle are axially slidable within a syringe barrel during use. The user may push and pull the distal end of the handle, causing corresponding axial movement of the mixing member within the syringe barrel. The devices and systems may be used to homogenously mix difficult to mix two part compositions, as well as single part compositions which include components that may tend to separate over time (e.g., a fluoride varnish composition).