Cyanobacteria have played a profound role in shaping the biosphere, most notably through the Great Oxygenation Event (GOE) with the advent of photosynthesis. Cyanobacteria also contribute to global primary production through biological nitrogen fixation (BNF) using nitrogenase, an oxygen-labile enzyme complex that evolutionarily predates the GOE. Current literature reports nitrogenase activity in unicellular cyanobacteria is protected from oxygen through a diurnal separation of photosynthesis and BNF. However, historic conditions of continuous-light and warm temperature at polar latitudes during the Triassic and Cretaceous may have created a selective advantage amongst unicellular cyanobacteria for non-temporal mechanisms of maintaining nitrogenase activity in the presence of oxygen. Here we report constitutive nitrogenase activity concurrent with a net-gain of oxygen through photosynthesis in a continuous-light adapted culture of the unicellular cyanobacteria, Cyanothece sp. ATCC 51142. Nitrogenase activity in the adapted culture exhibited dependence on light and increased resilience to artificially raised oxygen-tension compared to traditional culture. We predict cyanobacteria closely related to Cyanothece sp. ATCC 51142 also possess this physiology and found an accessory predicted proteome with functional relevance. This work provides a model of light-driven, oxygen-tolerant, constitutive nitrogenase activity and suggests this physiology may be conserved in closely related unicellular diazotrophic cyanobacteria with implications for primary production in polar ecosystems and potential biotechnological application in sustainable agriculture production.