A mixed-voltage input/output (I/O) buffer includes an output buffer circuit. The output buffer circuit includes an output stage circuit, a gate-tracking circuit and a floating N-well circuit. The output stage circuit includes stacked pull-up P-type transistors and stacked pull-down N-type transistors, in which a first P-type transistor of the stacked pull-up P-type transistors and a first N-type transistor of the stacked pull-down N-type transistors are coupled to an I/O pad. The gate-tracking circuit controls gate voltage of the first P-type transistor in accordance with a voltage of the I/O pad to prevent leakage current. The floating N-well circuit provides N-well voltages for an N-well of the first P-type transistor and an N-well of a second P-type transistor, controlling gate voltage of the first P-type transistor, of the gate-tracking circuit to prevent leakage current.