Subclinical Cushing syndrome is a condition of mild autonomous cortisol excess (MACE) that lacks typical features of Cushing syndrome but is associated with many complications. It represents a common hormonal dysfunction among patients with adrenal incidentaloma (AI), defined as unexpected masses or lesions found in the adrenal glands during radiological examinations of the chest or abdomen unrelated to adrenal gland assessment. The study evaluated the accuracy of dehydroepiandrosterone sulfate (DHEA-S) and dehydroepiandrosterone sulfate ratio (calculated by dividing the DHEA-S value by the age and sex-adjusted normal range of DHEA-S) in detecting MACE in AI patients. A cross-sectional study was conducted from April 2021 to July 2022 at the Faiha Specialized Diabetes, Endocrine, and Metabolism Centre (FDEMC) in Basrah, southern Iraq, involving 38 AI patients. Comprehensive laboratory and radiological evaluations were performed, including tests for adrenocorticotropic hormone (ACTH), renin, aldosterone, aldosterone/renin ratio (ARR), metanephrine, normetanephrine, cortisol, DHEA-S, and the 1-mg overnight dexamethasone suppression test (1-mg ONDST). Among the AI patients, 14% had MACE. Both DHEA-S ≤75 µg/dL and a DHEA-S ratio ≤1.7 exhibited a sensitivity of 80% each, with specificities of 73.3% and 76.6%, respectively, in diagnosing MACE in individuals aged ≤65 years. The negative predictive values were 95.7% and 95.8%, respectively. Low DHEA-S and DHEA-S ratio had high sensitivity and specificity in predicting MACE among AI patients aged ≤65 years, with strong negative predictive value.