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Interfacial modulation has been considered an effective strategy to enhance the thermoelectric (TE) performance by decoupling thermal and electrical parameters. However, inappropriate interfacial heterojunctions can significantly deteriorate electrical transport while enhancing phonon scattering, which causes no substantial improvement in TE performance. Herein, we introduce two-dimensional graphene or MXene into the Mg3.2Sb0.5Bi1.49Te0.01 matrix by ball mixing combined with rapid sintering to modify the composition and structure at the interface. The differences in morphology, size and distribution of interfacial graphene and MXene lead to distinct results which show that incorporated graphene flakes with larger size hinder the carrier transport, while the homogeneously dispersed tiny MXene provides improved carrier channels and boosts the mobility. On the other hand, such uniformly diffused heterogeneous MXene structures create stronger scattering on phonons compared to enriched graphene phases, resulting in a low lattice thermal conductivity of ∼0.66 W m−1 K−1 and enhanced zT of ∼1.0 at 513 K in Mg3.2Sb0.5Bi1.49Te0.01-0.6 wt% MXene with good component stability in air. This work highlights the importance of size, concentration, and distribution in the interfacial modulation strategy, demonstrating an effective method for developing high TE performance Mg3(Sb, Bi)2-based near-room-temperature materials. [ABSTRACT FROM AUTHOR]