PCDH19 epilepsy (DEE9) is an X linked epilepsy syndrome associated to cognitive and behavioural disturbances. Unlike in most other X linked diseases, heterozygous females are affected while hemizygous males are spared. It has been proposed that DEE9 pathogenesis is related to disturbed cell-to-cell communication associated with mosaicism and consequential erroneous network wiring. However, the effects of mosaic PCDH19 expression on cortical network activity are unknown. We mimicked the pathology of DEE9 by introducing a patch of mosaic protein expression in the cortex of conditional PCDH19 knockout mice. LFP recordings demonstrate transient episodes of hyperexcitability and disturbed slow wave activity – a crucial component of NREM sleep. These alterations were also observed if the mosaic patch was introduced in adult mice, demonstrating that PCDH19 is not merely a developmental disease. Our results indicate that a focal mosaic mutation of PCDH19 disrupts network cortical computation and broaden our current understanding of DEE9.