<p>Energy efficiency will contribute to environmental protection through the reduction of pollution and other externalities associated energy usage. The mitigation of externalities will contribute to environmental sustainability. In addition, the usage of renewable energy can stimulate energy efficiency but the required initial capital outlay poses a big constraint to most small and medium enterprises. The study investigated energy efficiency and environmental sustainability using green energy as a mediating variable. The purpose was to ascertain the extent to which energy efficiency influences sustainability and the extent to which green energy mediates the relationship between energy efficiency and sustainability. The design was a cross-sectional survey of 219 randomly selected respondents from 2 private universities in north central and southwestern Nigeria. The study used a structured questionnaire to collect data and the questionnaire was validated using convergent and divergent validities while composite technique served as the test for reliability. Path diagram analysis of the structural equation model was used to analyse the data. The results show that energy efficiency has a significant relationship with green energy and sustainability; and green energy mediates the relationship between energy efficiency and sustainability. The study makes a significant contribution to the extinction avoidance theory as well as contributes to empirical literature through the use of green energy to mediate the relationships between energy efficiency and sustainability.</p>