Purpose: This systematic review covers the current stage of research on subtle cognitive impairment with connected speech. It aims at surveying the linguistic features in use to single out those that can best identify patients with mild neurocognitive disorders (mNCDs), whose cognitive changes remain underdiagnosed. Method: We followed the Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines and proposed a full definition of features for the analysis of speech features. Fifty-one studies met the inclusion criteria. Most of them focused on age-related progressive diseases and included fewer than 30 subjects. Results: A total of 384 features labeled with 335 different names was retrieved, yielding various results in discriminating individuals with mNCDs from controls. Conclusions: This finding highlights the need for harmonized labels to further investigate mNCDs with linguistic markers. We suggest two different ways of assessing a feature's reliability. We also point out potential methodological issues that remain to be resolved, along with recommendations for reproducible research in the field.