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The detection of the amino acid glycine and its amine precursor methylamine on the comet 67P/Churyumov-Gerasimenko by the Rosetta mission provides strong evidence for a cosmic origin of amino acids on Earth. How and when such molecules form along the process of star formation remains debated. Here we report the laboratory detection of glycine formed in the solid phase through atom and radical–radical addition surface reactions under dark interstellar cloud conditions. Our experiments, supported by astrochemical models, suggest that glycine forms without the need for 'energetic' irradiation (such as ultraviolet photons and cosmic rays) in interstellar water-rich ices, where it remains preserved, during a much earlier star-formation stage than previously assumed. We also confirm that solid methylamine is an important side-reaction product. A prestellar formation of glycine on ice grains provides the basis for a complex and ubiquitous prebiotic chemistry in space enriching the chemical content of planet-forming material. © 2020, The Author(s), under exclusive licence to Springer Nature Limited. ; We thank T. Lamberts and I. Jiménez-Serra for stimulating discussions. This research was funded through a VICI grant of the NWO (the Netherlands Organization for Scientific Research) and A-ERC grant number 291141 (CHEMPLAN). Financial support from the Danish National Research Foundation through the Center of Excellence 'InterCat' (Grant agreement no. DNRF150) and from NOVA (the Netherlands Research School for Astronomy) and the Royal Netherlands Academy of Arts and Sciences (KNAW) through a professor prize is acknowledged. S.I. acknowledges the Royal Society for financial support through the University Research Fellowship (grant number UF130409), the University Research Fellowship Renewal 2019 (grant number URF\R\191018), the Research Fellows Enhancement Award (grant number RGF\EA\180306) and the Holland Research School for Molecular Chemistry (HRSMC) for a travel grant. G.F. acknowledges financial support from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie actions grant agreement number 664931 and support from an 'iALMA' grant (CUP C52I13000140001) approved by MIUR (Ministero dell'Istruzione, dell'Universitá e della Ricerca). A.R.C. and R.T.G. thank the NASA Astrophysics Research and Analysis Research programme for funding through grant number NNX15AG07G. V.K. was funded by the NWO PEPSci (Planetary and ExoPlanetary Science) programme. This work benefited from collaborations within the framework of the FP7 ITN LASSIE consortium (grant number GA238258).