Understanding the temporal variation in mosquito populations is essential for predicting and preventing vector-borne disease outbreaks. This prospective longitudinal study investigated the relationship between mosquito population density and the incidence of vector-borne diseases in urban and rural areas of Telangana, India, over two consecutive transmission seasons (June 2025–June 2026). As a primary field-based investigation, systematic entomological surveillance was conducted at 28 representative sites, and monthly mosquito density data were integrated with disease surveillance records obtained from regional health authorities. Disease incidence followed a similar seasonal trend with a 2-4-week lag, ranging from 0.8 to 32.8 cases per 100,000 population. A strong positive correlation was observed between mosquito density and disease incidence (Pearson's r = 0.94, p < 0.001), indicating that increases in vector abundance were closely associated with disease transmission. Culex quinquefasciatus was the predominant species (36.6%), followed by Aedes aegypti (21.4%) and Anopheles culicifacies and Anopheles stephensi (12.8%), with seasonal shifts in species dominance observed throughout the study period. Temperature and rainfall were identified as the principal environmental drivers of mosquito population dynamics, explaining approximately 52% of the seasonal variability in vector abundance. The observed lag between peak vector density and disease incidence highlights the potential of entomological surveillance as an effective early-warning system for predicting disease outbreaks. These findings emphasize the importance of integrating vector surveillance with climatic and epidemiological monitoring to support timely public health interventions and strengthen vector-borne disease control strategies in urban and rural regions of Telangana.