The crystal structure and magnetoresponsive properties of Fe-doped MnCoGe alloys have been investigated using x-ray diffraction (XRD) and magnetic measurements. By alloying the Fe-containing isostructure compounds into MnCoGe, a magnetostructural transition from paramagnetic austenite to ferromagnetic martensite with large magnetization difference can be realized in a temperature window between the Curie temperatures of the austenite and martensite, resulting in a magnetic-field-induced martensitic transformation and large magnetic-entropy changes. A structural and magnetic phase diagram of Fe-doped MnCoGe alloys is presented. ► Isostructural alloying can manipulate structural stability of Fe-doped MnCoGe alloys. ► A magnetostructural transition was realized in a temperature window between TCA and TCM. ► The remarkable magnetoresponsive properties in Fe-doped MnCoGe alloys were found. ► A structural and magnetic phase diagram of Fe-doped MnCoGe alloys was proposed.