AbstractAnthropogenic freshwater salinization affects thousands of lakes worldwide, and yet little is known about how salt loading may shift timing of lake stratification and spring mixing in dimictic lakes. Here, we investigate the impact of salinization on mixing in Lakes Mendota and Monona, Wisconsin, by deploying under‐ice buoys to record salinity gradients, using an analytical approach to quantify salinity thresholds that prevent spring mixing, and running an ensemble of vertical one‐dimensional hydrodynamic lake models (GLM, GOTM, and Simstrat) to investigate the long‐term impact of winter salt loading on mixing and stratification. We found that spring salinity gradients between surface and bottom waters persist up to a month after ice‐off, and that theory predicts a salinity gradient of 1.3–1.4 g kg⁻¹ would prevent spring mixing. Numerical models project that salt loading delays spring mixing and increases water column stability, with ramifications for oxygenation of bottom waters, biogeochemistry, and lake habitability.