The long-term (1991–2010) monitoring data for cadmium (Cd) collected from seven major cities in Korea were evaluated to assess spatial-temporal trends in urban areas and help improve air quality control. The results show that the level of Cd pollution in the different cities, as distinguished by industrial activity, was the highest in Ulsan (9.21 ± 6.29 ng/m3) and Incheon (6.25 ± 6.05 ng/m3). During the study period, Cd concentrations showed significant reductions in all monitoring cities, especially in the more industrialized ones (e.g., a nearly six-fold reduction in Ulsan over 13 years), while in other areas this reduction was more moderate (about 1.7-fold). In addition, Cd concentrations generally peaked in the winter, with the exception of the more industrialized areas. Although Cd levels in Ulsan and Incheon exceeded the general guidelines of 5 ng/m3 (e.g., those of the US EPA) until recently (e.g., 2004), its values in most urban areas in Korea have been continuously decreasing over the 20-year period examined in this work. Therefore, greater efforts are necessary to more effectively reduce Cd released from the prominent source activities in the urban environments.