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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. [ABSTRACT FROM AUTHOR]