Abstract CUPID-Mo, located in the Laboratoire Souter rain de Modane (France), was a demonstrator for the next generation 0νββ decay experiment, CUPID. It consisted of an array of 20 enriched Li2 100MoO4 bolometers and 20 Ge light detectors and has demonstrated that the technology of scintillating bolometers with particle identification capabilities is mature. Furthermore, CUPID-Mo can inform and validate the background prediction for CUPID. In this paper, we present a detailed model of the CUPID-Mo backgrounds. This model is able to describe well the features of the experimental data and enables studies of the 2νββ decay and other processes with high precision. We also measure the radio-purity of the Li2 100MoO4 crystals which are found to be sufficient for the CUPID goals. Finally, we also obtain a background index in the region of interest of 3.7 +0.9 −0.8 (stat)+1.5 −0.7 (syst)×10−3 counts/ΔEFWHM/moliso/year, the lowest in a bolometric 0νββ decay experiment.