Abstract: Multi-principal-element alloys (MPEAs) have emerged as a transformative class of metallic materials, surpassing conventional alloys due to their “four core effects.” The inherent compositional complexity and programmable multifunctionality of MPEAs collectively drive their emergence as a vanguard in materials innovation. By synergistically modulating metastable engineering and magneto-volume effects, we developed a MPEA (Fe,Co,Cr)_100−xNi_x with an ultralow coefficient of thermal expansion (α_l = 1.00 × 10⁻⁶ K⁻¹, 100–400 K) and exceptional mechanical properties (tensile strength: 560 MPa, the elongation to failure: 53%). This alloy exhibits both significant transformations induced plasticity (TRIP) and zero thermal expansion effects (Invar) at room temperature, classified as a recently proposed TRIP-Invar alloy. In situ magnetic analysis reveals that ferromagnetic order mediates pronounced magnetic compensation of intrinsic lattice contraction during cooling through spin-state transitions, thereby generating zero thermal expansion behavior. In situ neutron diffraction reveals that the good strength–plasticity trade-off arises from a deformation-triggered martensitic transformation, which enhances strain hardening through dislocation multiplication and grain boundary reinforcement. This work proposes a materials design strategy for next-generation structural-functional integrated materials, advancing the fundamental understanding of thermal expansion-mechanical property optimization in MPEAs.