Background: The aim of the study was to describe distinct electroencephalogram (EEG) phenotypes defined after routine visual EEG analysis in a large memory clinic cohort and to investigate their relationship to cerebrospinal fluid (CSF) biomarkers. Methods: Patients with Alzheimer's disease (n = 131), mild cognitive impairment (n = 285), subjective cognitive impairment (n = 310), and mixed dementia (n = 29) were assessed clinically with neuroimaging, EEG and CSF investigations. EEG phenotypes were based on frequency of background activity (BA) and presence and degree of episodic abnormalities (EA). Results: BA and EA differed significantly (p < 0.001) between diagnostic groups. A lower CSF amyloid β42/phospho-tau ratio and higher total tau were associated with slower BA (p < 0.01) and a higher degree of EA (p < 0.04). Conclusions: Slowing of BA in combination with EA seems to be related to biological markers of neurodegeneration.