Standardized diet compositions for 108 species of marine mammals were derived from published accounts of stomach contents and morphological, behavioural and other information. Based on the trophic levels (TL) of the eight types of food items used to describe the diet compositions (previously estimated using the ECOPATH II approach and software, applied to over 50 marine ecosystems) TLs were estimated for each marine mammal species, and compared with published estimates derived using stable isotope ratios. We anticipate that these TL estimates and ancillary statistics that may be derived from the data herein may be of use for ecosystem modelling and similar purposes.