Low-expansion alloys are essential structural-functional materials for advanced technologies requiring stringent dimensional stability. They are key components in precision metrology, electronic and microwave devices, cryogenic systems, and ultraprecision manufacturing equipments, where thermal deformation must be strictly controlled. However, conventional Fe-Ni Invar alloys possess insufficient mechanical strength despite their exceptionally low coefficient of thermal expansion, which limits their applicability in load-bearing environments. Design concept of high-entropy alloys offer a promising pathway to overcome this limitation through multiprinciple element alloy design and the associated synergistic effects. In this work, a novel low-expansion alloy, Al₁Cr₁(Fe₆₅Co₄Ni₃₁)₉₈, was developed by introducing Al and Cr into the multicomponent system and applying thermomechanical processing to tailor and refine its microstructure. This design strategy aims to achieve the synergistic optimization of thermal expansion behavior and mechanical performance. Additionally, in situ XRD during heating was employed to elucidate the underlying mechanism and monitor phase evolution. After thermomechanical processing, the alloy exhibited pronounced grain refinement and an increased martensite volume fraction of 8.91%. The microstructure further contained abundant deformation twins and a high density of lattice defects, which collectively enhanced the mechanical strength and thermal stability. Within the temperature range of -60 ^oC to 100 ^oC, the coefficient of thermal expansion decreased to 1.10\times10^-6-2.04\times10^-6oC^-1. In addition to the Invar effect, this ultralow expansion behavior is attributed to the partial compensation of lattice thermal vibrations by the volume contraction associated with martensite reduction during heating, together with the suppression of anharmonic lattice vibrations induced by interfaces and defects. Meanwhile, the refined microstructure delivered an excellent combination of strength and ductility, achieving a yield strength of 324 MPa, an ultimate tensile strength of 452 MPa, and a fracture elongation greater than 20%. Compared with conventional Invar alloys, the designed alloy exhibited a higher specific strength while maintaining a low coefficient of thermal expansion. These results demonstrate that the synergistic optimization of compositional design and thermomechanical processing enables the exceptional integration of low thermal expansion with robust mechanical properties, offering valuable guidance for developing dimensionally stable structural alloys.