A systematic screening program for detecting autism at 18 months was progressively implemented in Oman since 2017. The primary objective of this study was to examine whether systematic use of M-CHAT-R/F screening is associated with lowering the age at ASD diagnosis, controlling for other predictors of age at ASD diagnosis. The study is based on a cross-sectional retrospective review of data extracted from electronic records of 756 children diagnosed with ASD between 1st January 2017 and 30th June 2023. Descriptive statistics and multiple linear regression models were used for data analysis. Of 756 children (mean age: 7.3 years, age range: 1.8–14 years; 77.5% males) included in this study, 98 (13%) underwent M-CHAT-R/F screening. The average age at ASD diagnosis was 60.7 months (95% CI: 58.8, 62.7 months). A significantly lower age at ASD diagnosis was observed among children who underwent M-CHAT-R/F screening compared to those who did not (39.4 vs. 63.8 months; p < 0.001). Multiple regression analysis indicates that children who were screened with M-CHAT-R/F received a diagnosis of ASD at an average age 20% earlier than children who were not screened with M-CHAT-R/F (adjOR = 0.80; 95% CI: 0.73, 0.88, p < 0.001). The population-based M-CHAT-R/F screening program appears to have reduced the average age of diagnosis of ASD among children. The findings may translate into earlier access to intervention and possibly improved long-term outcomes for children with ASD.