The blending of hydrogen with natural gas (NG) is proposed as the simplest short-term solution for storing, transporting, and using hydrogen. The composition of hydrogen-enriched NG (HENG) blends strongly affects their thermo-physical properties and may influence the mechanical stability of the NG pipelines used. Therefore, its quantification is essential but this is hampered due to the current lack of adequate analytical techniques. Here, we introduce for the first time high-pressure (HP) benchtop proton NMR spectroscopy as a quantitative method for simple and low-cost analysis of pressurized HENG blends. For implementing the method towards an online monitoring scenario, an inexpensive HP flow-through cell, capable of operating up to 245 bar, was designed and tested. The capability of the NMR method was demonstrated on selected blends with different amounts of hydrogen and by in-situ monitoring the change in the composition of a blend with increasing storing time due to the buoyancy of hydrogen.