In-situ NMR spectroscopy can provide valuable insight into chemical reactions, including detailed information on reaction kinetics and thus mechanism. The application of in-situ NMR techniques to heterogeneous reactions has so far been limited by the requirement for efficient mixing, which is challenging within the confines of a standard NMR spectrometer. We report an automated plunger-based device for the mixing of heterogeneous solid-liquid reactions conducted within a 5 mm tube in a standard NMR probe, allowing reaction kinetics and speciation to be monitored in-situ. The system has been tested by measuring the kinetics for three diverse reactions and comparison of these with the reaction velocities measured by routine ex-situ analysis after sampling. The effect of key parameters including plunger design, plunger velocity, solid settling time, and mixing-to-acquisition time ratio are explored to determine the optimum parameters for combining efficient mixing with spectral quality. With adaptation, the device may also be suitable for benchtop NMR.