Background: Li-Fraumeni Syndrome (LFS) is an autosomal dominant cancer susceptibility syndrome due to germline pathogenic variants in TP53 and is associated with a significant risk for sarcomas, female breast, and other cancers. A subset of individuals undergoing germline TP53 analysis will have a variant with an allele fraction (AF) less than that in heterozygous individuals, consistent with mosaicism. TP53 mosaicism may be limited to hematopoietic cells, such as in age-related hematopoietic expansion, or may be constitutional, in which case the variant exists in additional tissues and may increase LFS-associated cancer risks. This study aims to characterize the clinical characteristics and follow-up results of cases with a TP53 mosaic pathogenic or likely pathogenic variant (PV). Methods: We performed a retrospective review of all individuals undergoing multi-gene hereditary cancer panel testing at our diagnostic laboratory. Cases included those with mosaicism for one or more TP53 PV, characterized as an AF of < 35.0% on next-generation sequencing (NGS) on a blood or oral rinse specimen. Data were analyzed utilizing descriptive statistics and hypothesis tests, including Fisher?s exact test. Results: We identified 117 TP53 mosaic PV cases. Among 37 cases that had one or more relatives undergo testing for the mosaic PV, no relatives were positive. Among 21 cases that underwent subsequent fibroblast (FB) testing, 19.0% (4/21) correlated with the initial mosaic result, suggesting constitutional mosaicism. Of those that confirmed on FB, the mean AF for the original sample was 28.3% (n = 4; SD = 3.2%), whereas the mean for those negative on FB was 18.5% (n = 17; SD = 6.3%). Confirmation of mosaicism on FB was associated with an initial NGS AF ≥25.0% (p = 0.0276; FB Pos: 3/4, FB Neg: 2/17) and with a personal history of a breast cancer or sarcoma diagnosed < 46 years (p = 0.0276; FB Pos: 3/4, FB Neg: 2/17). Conclusions: Although NGS is not purely quantitative, apparent constitutional mosaicism appears to correlate with higher AF on NGS. In addition, phenotypes of cases with TP53 mosaicism confirmed via FB are more suggestive of LFS. Further studies are required to establish clinical correlation.