Diabetes mellitus (DM) is a major health issue in Saudi Arabia. Prevention of DM and its complications requires an understanding of the disease and modifiable behaviors (e.g., physical activity—PA). The purpose of this study was to examine the trends in knowledge of the general population regarding DM to better understand the shortcomings in the current awareness programs. This article presents a cross-sectional series study where a survey was distributed to a total of 3493 participants over four years, from 2017 till 2020, to assess general knowledge about DM, including information about PA. The mean percentage of correct responses of DM general knowledge was 63.8 ± 19.0 in 2017, which decreased to 61.3 ± 18.7 in 2020 with a significant beta coefficient of −0.8 ± 0.2 (p < 0.001). Participants’ awareness about PA remained constantly high for four years: the mean percentage of correct responses was 82.1 ± 23.6 in 2017 and 82.0 ± 23.1 in 2020, and the beta coefficient was −0.5 ± 0.3 (p = 0.147). Furthermore, stratification by demographics showed that the majority of the subgroups (age, sex, educational status, marital status, having relative with DM, nationality) reported a significant declining trend in general DM knowledge. In addition, some of the subgroups also showed a declining trend in PA awareness. Future prevention efforts should assess the community’s DM knowledge regularly to tailor awareness efforts to the population segments that need heightened educational interventions.