To control adherence to age limits regarding sales of tobacco products, Swedish authorities can conduct compliance checks. Compliance checks involve prior information to all retailers, mystery shopping, and subsequent feedback to the retailers. This study investigated whether compliance checks can decrease the rates of cigarette sales to underaged adolescents. Test purchases of cigarettes were conducted using pseudo-underaged mystery shoppers, i.e., 18-year-old adolescents with a younger appearance not carrying ID, to measure the refusal rate and rate of ID checks. Test purchases were conducted at 257 retail outlets in 13 municipalities in Stockholm County at baseline 2017 and follow-up 2019, respectively. In between the measurements, six municipalities (intervention area) conducted compliance checks, and seven municipalities were used as a comparison. Comparing baseline and follow-up, rates of refusal (70.4 to 95.8%) and ID checks (80.3 to 95.8%) improved in the intervention area. In the comparison area, refusal rates increased (80.9 to 85.2%), and ID check rates remained stable (at 86.1%). Significant group × time interaction effects reveal that the rates of refusal and ID checks differently changed in the study areas over time. These results indicate that compliance checks are an effective method to decrease cigarette sales to underaged adolescents.