The food-borne pathogen Listeria (L.) monocytogenes is able to survive for months and even years in food production environments. Strains belonging to sequence type (ST)121 are particularly found to be abundant and to persist in food and food production environments. To elucidate genetic determinants characteristic for L. monocytogenes ST121, we sequenced the genomes of 14 ST121 strains and compared them with currently available L. monocytogenes ST121 genomes. In total, we analyzed 70 ST121 genomes deriving from 16 different countries, different years of isolation, and different origins—including food, animal and human ST121 isolates. All ST121 genomes show a high degree of conservation sharing at least 99.7% average nucleotide identity. The main differences between the strains were found in prophage content and prophage conservation. We also detected distinct highly conserved subtypes of prophages inserted at the same genomic locus. While some of the prophages showed more than 99.9% similarity between strains from different sources and years, other prophages showed a higher level of diversity. 81.4% of the strains harbored virtually identical plasmids. 97.1% of the ST121 strains contain a truncated internalin A (inlA) gene. Only one of the seven human ST121 isolates encodes a full-length inlA gene, illustrating the need of better understanding their survival and virulence mechanisms.