The martensitic and magnetic phase transformations in MnNiGe1-xSnx (0 ≤ x ≤ 0.200) alloys were investigated using X-ray diffraction, differential thermal analysis, and magnetization measurements. Results indicate that increasing the Sn substitution in MnNiGe1-xSnx results in 1) a decrease of the martensitic transformation temperature from 460 to 100 K and 2) a conversion of spiral antiferromagnetic (AFM) to antiparallel AFM structure in martensite. Based on these features, a remarkable magnetic-field-induced paramagnetic/spiral AFM and FM/AFM magnetostructural transformations are found, and large positive and negative magnetocaloric effects are obtained. The magnetoresponsive effects of MnNiGe1-xSnx alloys are enhanced by Sn substitution. A structural and magnetic phase diagram of MnNiGe1-xSnx alloys has been proposed.