Thermomyces lanuginosus lipase (TlL) and related lipases become activated in low-polarity environments that exist at the water-lipid interface where a structural change of the "lid" region occurs. In this work, we have investigated the activation of TlL (Lipase_W89) and certain lid mutants, containing either a single positive charge mutation, E87K (Lipase_K87_W89), within the lid region or a lid residue composition of both lipase and esterase character (Hybrid_W89) as a function of solvent polarity. Activation differences between the variants and TlL were studied by a combination of biophysical and theoretical methods. To investigate the structural changes taking place in the lid region upon lipase activation, we used a fluorescence-based method measuring the efficiency of Trp89 in the lid to quench the fluorescence of a bimane molecule attached in front (C255) and behind (C61) the lid. These structural changes were compared to the enzymatic activity of each variant at the water-substrate interface and to theoretical calculations of the energies associated with lid opening as a function of the dielectric constant (ε) of the environment. Our results show that the lid in Lipase_K87_W89 undergoes a pronounced structural transition toward an open conformation around ε = 50, whereas only small changes are detected for Lipase_W89 ascribed to the stabilizing effect of the positive charge mutation on the open lid conformation. Interestingly, Hybrid_W89, with the same charge as Lipase_W89, shows a stabilization of the open lid even more pronounced at high solvent polarities than that of Lipase_K87_W89, allowing activation at ε < 80. This is further indicated by measurement of the lipase activity for each variant showing that Hybrid_W89 is more quickly activated at the water-lipid interface of a true, natural substrate. Combined, we show that a correlation exists between structural changes and enzymatic activities detected on one hand and theoretical calculations on lid opening energies on the other. These results highlight the key role that the lid plays in determining the polarity-dependent activation of lipases.