Monolayer V$_4$C$_3$T$_z$ is synthesized and its atomic structure is studied using high-end transmission electron microscopy. X-Ray diffraction reveals key 3D to 2D crystal transformations as a V$_4$AlC$_3$ crystal is transformed into V$_4$C$_3$T$_z$ in synthesis processes. The charge storage properties of V$_4$C$_3$T$_z$ film electrodes are investigated for supercapacitor applications in 3 M H$_2$SO$_4$. V$_4$C$_3$T$_z$ film electrodes shows an excellent capacitance of up to 469.6 F g$^{-1}$ and 845.7 F cm$^{-3}$, rate performance up to 30 A g$^{-1}$ and cycling stability up to 10,000 cycles. A combination of electrochemical kinetics/mass transport models, staircase potentio-electrochemical impedance spectroscopy and in situ X-Ray absorption spectroscopy reveals, for the first time for this MXene, the underlying charge storage mechanisms, consisting of double layer capacitance, pseudocapacitance and a minor contribution from mass transport-controlled processes. The latter two implying a outstanding redox activity superior to Ti-based MXenes. The stability in standard environments, mechanical flexibility and the demonstrated excellent charge storage performance of V$_4$C$_3$T$_z$ makes it one of the best candidates for supercapacitor applications, especially in miniaturized devices.