The development of therapeutic interventions for hearing loss requires a detailed understanding of the genes and proteins involved in hearing. The FOXG1 protein plays an important role in early neural development and in a variety of neurodevelopmental disorders. Previous studies have shown that there are severe deformities in the inner ear in Foxg1 knockout mice, but due to the postnatal lethality of Foxg1 knockout mice, the role of FOXG1 in hair cell (HC) development and survival during the postnatal period has not been investigated. In this study, we took advantage of transgenic mice that have a specific knockout of Foxg1 in HCs, thus allowing us to explore the role of FOXG1 in postnatal HC development and survival. In the Foxg1 conditional knockout (CKO) HCs, an extra row of HCs appeared in the apical turn of the cochlea and some parts of the middle turn at postnatal day (P)1 and P7; however, these HCs gradually underwent apoptosis, and the HC number was significantly decreased by P21. Auditory brainstem response tests showed that the Foxg1 CKO mice had lost their hearing by P30. The RNA-Seq results and the qPCR verification both showed that the Wnt, Notch, IGF, EGF, and Hippo signaling pathways were down-regulated in the HCs of Foxg1 CKO mice. The significant down-regulation of the Notch signaling pathway might be the reason for the increased numbers of HCs in the cochleae of Foxg1 CKO mice at P1 and P7, while the down-regulation of the Wnt, IGF, and EGF signaling pathways might lead to subsequent HC apoptosis. Together, these results indicate that knockout of Foxg1 induces an extra row of HCs via Notch signaling inhibition and induces subsequent apoptosis of these HCs by inhibiting the Wnt, IGF, and EGF signaling pathways. This study thus provides new evidence for the function and mechanism of FOXG1 in HC development and survival in mice.