In this paper, in-situ high-energy X-ray diffraction was employed to trace the phase transition via the change in the lattice strains under multiple (stress and magnetic) fields for a polymer-bonded Ni–Co–Mn–In composite. Under uniaxial compressive deformation, the parent phase in the Ni–Co–Mn–In alloy endured a compressive internal stress along the loading direction and a tensile internal stress in the transverse direction, leading to the formation of a highly textured martensite. This paper is intended to examine the effect of the external magnetic field on the phase transition in two types of martensite, i.e. highly textured martensite and temperature-induced self-accommodated martensite. For the highly textured martensite, a compressive or tensile internal stress can be observed for the parent phase parallel or perpendicular respectively to the preloading direction under an applied magnetic field. This is due to the magnetic-field-induced transformation from the highly textured martensite to the parent phase. For the self-accommodated martensite accompanied by the random distribution of martensitic variants, no obvious internal stresses are generated in the parent phase under an applied magnetic field of up to 6 T. The insight into the stress state of the alloy phase reinforces the in-depth understanding of the role of external fields in affecting the functional performance of the polymer-bonded Ni–Co–Mn–In composite.