Synaptic dysfunction is the best anatomical correlate of early cognitive impairment in Alzheimer's disease (AD). Electroencephalography (EEG) directly reflects brain electrical activity at the level of synapses. The aim of the present study was to investigate correlations of quantitative EEG measures, global field power (GFP) and global field synchronization (GFS), with conventional cerebrospinal fluid (CSF) biomarkers of neurodegeneration in patients diagnosed with subjective cognitive decline (n = 210), mild cognitive impairment (n = 230), and AD (n = 197). Decreased CSF amyloid β42 significantly correlated with increased theta and delta GFP, whereas increased p- and t-tau with decreased alpha and beta GFP. Decreased CSF amyloid β42 and increased p- and t-tau were significantly associated with decreased GFS alpha and beta. Subanalysis of the separate diagnostic groups demonstrated significant correlations between CSF biomarkers and generalized power and synchronization already in the subjective cognitive decline and mild cognitive impairment group. These results provide evidence that quantitative EEG measures are associated and possibly sensitive to distinct AD-like CSF biomarker profiles in cognitively impaired patients and are therefore promising early noninvasive markers of AD.