Objective: This study aimed to translate the General Medication Adherence Scale (GMAS) into English language and validate it in patients suffering from chronic illnesses. Methods: A 1-month study (January 2018) was conducted in a random sample of patients suffering from chronic illnesses who visited the outpatient departments of four tertiary healthcare facilities in Karachi, Pakistan. Translation of the tool and its content, as well as face validity, was carried out. Factor structure was explored (i.e. exploratory and partial confirmatory factor analyses were carried out) and fit indices were calculated for model fitting. Test-re-test reliability and internal consistency were analyzed. Validity of GMAS-English was established by convergent, discriminant, and concurrent validity analysis. Sensitivity analysis was conducted. Data was analyzed through SPSS version 23. The study was ethically approved by concerned authorities (Letter# NOV:15). Results: The GMAS was translated into English language by standard procedure. Factor analysis indicated a 3-factor model. Fit indices, namely normed fit index, Tucker Lewis index, comparative fit index, and root mean square of error approximation, were calculated with satisfactory results (i.e. NFI, TLI, and CFI > 0.9 and RMSEA < 0.08). Internal consistency (α) was 0.82. A high response rate of 91.6% was reported. GMAS-English established convergent, discriminant, and concurrent validities. The tool was sensitive (>75%) in screening patients with partial-to-low adherence based on their education level. Conclusion: The tool was translated in English language and demonstrated adequate internal consistency. The results indicate that GMAS-English is a valid and reliable tool to measure medication adherence in patients with chronic illness.