Cities are generally hotter than surrounding rural areas, forming what is also known as the Urban Heat Island (UHI) effect. These increases in temperature advance the phenology of many insects, including mosquitoes in the spring and therefore have the potential to increase disease risk in urban settings. However, research determining how these increased temperatures affect seasonal responses later in the season has been limited. In the U.S., the Northern house mosquito, Culex pipiens, is highly abundant in cities and is the primary vector of West Nile virus (WNV). Females of this species enter a state of developmental arrest, or diapause, in response to short days and low temperatures during autumn. Diapausing females cease reproducing and blood-feeding, and instead accumulate fat, thereby halting disease transmission. We hypothesized that increases in temperatures associated with the UHI effect during autumn day lengths may inhibit diapause initiation. To test this hypothesis, we reared mosquitoes environmental chambers with short day lengths to induce diapause, and various temperatures that simulated the UHI effect in temperate biomes during winter (20°C, 22°C, and 24°C). We then measured the fat accumulation, ovarian development, and blood-feeding proclivity of adult female mosquitoes. We found that mosquitoes reared at simulated UHI temperatures had the same level of fat, but that these females had larger egg follicles and were more likely to blood-feed. These data indicate that the UHI effect has the potential to interfere with normal seasonal responses in Cx. pipiens mosquitos. These results will contribute to growing knowledge on how the UHI affects public health and provide important information for predicting disease risk and managing mosquitoes for city residents.