5-methylcytosine (m5C) is a prevalent base modification in tRNA and rRNA but it also occurs more broadly in the transcriptome, including in mRNA. In pursuit of potential links of m5C with mRNA translation, we performed polysome profiling of human HeLa cell lysates and subjected RNA from resultant fractions to efficient bisulfite conversion followed by RNA sequencing (bsRNA-seq). Bioinformatic filters for rigorous site calling were devised to reduce technical noise. We obtained ~1,000 candidate m5C sites in the wider transcriptome, most of which were found in exonic regions of mRNA. Multiple novel sites were validated by amplicon-specific bsRNA-seq in independent samples of either human HeLa, LNCap and PrEC cells. Furthermore, RNAi-mediated depletion of either the NSUN2 or TRDMT1 m5C:RNA methyltransferases showed a clear dependence on NSUN2 for the majority of tested sites in mRNA and noncoding RNA. Candidate m5C sites in mRNAs are enriched in 5’UTRs and near start codons, and commonly embedded in a local context reminiscent of the NSUN2-dependent m5C sites found in the variable loop of tRNA. Analysing mRNA sites across the polysome profile revealed that non-conversion levels, at bulk and for many individual sites, were inversely correlated with ribosome association. Altogether, these findings emphasise the major role of NSUN2 in making this mark transcriptome-wide and further substantiate a functional interdependence of cytosine methylation level with mRNA translation.