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<i>Staphylococcus epidermidis</i> is found naturally on the skin but is a common cause of persistent orthopaedic device-related infections (ODRIs). This study used a pan-genome and gene-by-gene approach to analyse the clonality of whole genome sequences (WGS) of 115 <i>S. epidermidis</i> isolates from 55 patients with persistent ODRIs. Analysis of the 522 gene core genome revealed that the isolates clustered into three clades, and MLST analysis showed that 83% of the isolates belonged to clonal complex 2 (CC2). Analysis also found 13 isolate pairs had different MLST types and less than 70% similarity within the genes; hence, these were defined as re-infection by a different <i>S. epidermidis</i> strain. Comparison of allelic diversity in the remaining 102 isolates (49 patients) revealed that 6 patients had microevolved infections (>7 allele differences), and only 37 patients (77 isolates) had a ‘true’ persistent infection. Analysis of the core genomes of isolate pairs from 37 patients found 110/841 genes had variations; mainly in metabolism associated genes. The accessory genome consisted of 2936 genes; with an average size of 1515 genes. To conclude, this study demonstrates the advantage of using WGS for identifying the accuracy of a persistent infection diagnosis. Hence, persistent infections can be defined as ‘true’ persistent infections if the core genome of paired isolates has ≤7 allele differences; microevolved persistent infection if the paired isolates have >7 allele differences but same MLST type; and polyclonal if they are the same species but a different MLST type.