Tests measuring proactive semantic interference as The Loewenstein-Acevedo Scales for Semantic Interference and Learning (LASSI-L), has shown promising diagnostic properties for the diagnosis of Mild Cognitive Impairment (MCI) and dementia. LASSI-L may also be efficient in predicting cognitive decline in at-risk individuals. There is an unmet need to examine the diagnostic properties of the LASSI-L in a Danish context where traditional neuropsychological tests are typically applied when diagnosing possible dementia disorders. To investigate the reliability, convergent validity, and predictive value of the new Danish LASSI-L version in aMCI and mild dementia due to Alzheimer’s disease (AD). From a memory clinic we included 17 aMCI patients, 15 patients with mild dementia (AD), 17 patients with Subjective Cognitive Decline (SCD), and 30 healthy controls. Neuropsychological assessment was applied in all patients, and biomarker analyses were performed for patients with aMCI and mild AD. Cronbach’s alpha was 0.94. Patients with aMCI and mild dementia differed significantly from healthy controls on all LASSI-L measures. ROC analyses showed a very high AUC value for both patients with aMCI [0.85–0.97] and mild dementia [0.93–0.99]. SCD patients generally did not differ from controls, except for significantly lower scores on one item (Cued Recall A1) LASSI-L had high reliability and promising predictive value in the diagnosis of aMCI and mild AD due to AD. SCD patients diagnosed in a memory clinic did not differ significantly from healthy on the LASSI-L.