As part of the Core for Life Proteomics Working group, we have undertaken a four-year harmonization study to enhance the comparability of quality control results, identifying community reference values, intra-laboratory performance drifts, sources of heterogeneity, and improvement opportunities. The outcomes include a new quality control standard for longitudinal assessment of liquid chromatography and mass spectrometry system performance, measures of intra- and inter-laboratory variability, community reference values, and a 4-year longitudinal series of quality control data from multiple instruments and laboratories.