Mobile genetic elements (MGE) are critical yet understudied determinants of gut microbiome composition. In this secondary analysis of a randomized controlled trial (NCT06030713), we characterized the gut virome and plasmidome in 195 samples from 28 mother-infant dyads delivered by cesarean section. Infant mobilome increases in richness over the first 6 postnatal weeks, demonstrating high individual-specificity and temporal stability, establishing a personal persistent mobilome. Formula-fed infants exhibit greater mobilome richness than breastfed infants, with plasmid composition being influenced by antibiotic exposure and birth weight. Plasmids constitute a reservoir of antibiotic resistance genes (ARG), with around 5% of infant gut plasmid taxonomic units carrying ARG. Notably, ARG profiles do not differ with antibiotic exposure at birth. Mother-infant sharing of viral and plasmid strains primarily occurs after 6 months of age. Overall, our integrative analysis offers insights into the dynamics, modulation, and origin of MGE in the developing gut microbiome.