A family of 2D tetrathiafulvalene tetracarboxylate (TTFTC)-based lanthanide (Ln^III = Dy ^III, Er ^III, and Yb ^III) metal–organic frameworks (MOFs) is reported as a robust platform for mixed proton–electron conduction. Electronic transport arises from partially oxidized, π-stacked TTFTC columns with a fixed hole population imposed by framework stoichiometry, leading to narrow-band semiconducting behavior. The charge distribution within the π-stacks is preserved over a wide humidity range, resulting in only minor, compound-dependent variations in electronic conductivity. In parallel, proton transport is promoted by dense hydrogen-bond networks formed by coordinated water molecules anchored to rigid Ln₆ cluster walls, enabling an efficient proton conduction that remains operative across a broad range of humidity and temperature conditions (E_a < 0.4 eV at RH 80%), consistent with a dominant Grotthuss-type mechanism. Among the series, Er₆TTFTC₅ displays the highest room-temperature single-crystal electronic conductivity reported to date for a TTF-based MOF (1.0 × 10^–2 S cm^–1, 4-probe), together with a proton conductivity of 3.7 × 10^–3 S cm^–1 under humid conditions (80 °C, 80% RH). To enable direct comparison, both electronic and proton conductivities were evaluated under identical conditions, and an effective ambipolar conductivity (σ_amb) was derived from the independently measured contributions. The resulting values rank among the highest reported for structurally defined proton–electron mixed conductors (2.3 × 10^–3, 7.2 × 10^–4, and 5.9 × 10^–4 S cm^–1 at 80% RH and 80 °C for Er₆TTFTC₅, Yb₆TTFTC₅, and Dy₆TTFTC₅). Subtle variations across the lanthanide series are attributed to differences in framework rigidity and intermolecular interactions, which modulate transport properties without altering the underlying, structurally encoded conduction pathways.