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Mosaicism is the presence of two or more genetically distinct cell lineages originating from a single zygote. The skin frequently marks mosaic conditions through migration patterns of a population of mutant cells during embryogenesis. Somatic mutations in genes of the PI3K/PTEN/AKT/TSC/mTORC1 signaling pathway can result in segmental overgrowth, hamartomas, and malignant tumors, given the crucial role of this axis in cell growth. Mosaicism for activating mutations in AKT1 and PIK3CA is responsible for Proteus syndrome and PIK3CA-Related Overgrowth Spectrum, respectively. These frequently exemplify Happle’s patterns of cutaneous mosaicism. Postzygotic mutations in PTEN and TSC1/TSC2 result in mosaic forms of the PTEN Hamartoma Tumor Syndrome and tuberous sclerosis complex, which may present as disseminated or segmental disease. Distinct features observed in these mosaic conditions may be attributed to differences in embryological timing or tissue type harboring the mutant cells. Deep sequencing methods of affected tissue is often necessary to diagnosis these disorders. Oral mTORC1 inhibitors, such as sirolimus and everolimus, are useful for treating tuberous sclerosis complex, and drugs targeting mTORC1 or other points along this signaling pathway are in clinical trials to treat several of these disorders.