AbstractWe present an alternative method to determine leaf CO₂ assimilation rate (A_n), eliminating the need for gas exchange measurements in proximal and remote sensing. This method combines the Farquhar–von Caemmerer–Berry photosynthesis model with mechanistic light reaction (MLR) theory and leaf energy balance (EB) analysis. The MLR theory estimates the actual electron transport rate (J) by leveraging chlorophyll fluorescence via pulse amplitude‐modulated fluorometry for proximal sensing or sun‐induced chlorophyll fluorescence measurements for remote sensing, along with spectral reflectance. The EB equation is used to directly estimate stomatal conductance from leaf temperature. In wheat and soybean, the MLR‐EB model successfully estimated A_n variations, including midday depression, under various environmental and phenological conditions. Sensitivity analysis revealed that the leaf boundary layer conductance (g_b) played an equal, if not more, crucial role compared to the variables for J. This was primarily caused by the indirect influence of g_b through the EB equation rather than its direct impact on convective CO₂ exchange on the leaf. Although the MLR‐EB model requires an accurate estimation of g_b, it can potentially reduce uncertainties and enhance applicability in photosynthesis assessment when gas exchange measurements are unavailable.