In magnetostrictive RFe2 (R – rare earth) materials, magnetic properties such as magnetocrystalline anisotropy and Curie temperature can have a profound impact on the magnetostriction, often reducing its value in practically applicable magnetic field at normal conditions. Tuning the atomic structure by Mn alloying is one of the research strategies that allows to improve both elastic and magnetoelastic properties. Here we investigate novel non-stoichiometric TbFe2Mnx (0 ≤ x ≤ 0.25) compounds in which Mn partially replaces both Tb and Fe. Curie temperature and magnetic moment values decrease with manganese alloying. We interpret these effects as a consequence of lowered Fe-Fe exchange interactions which is confirmed within two-sublattice molecular field model. Magnetocrystalline anisotropy estimated by law of approach to magnetic saturation in high pulsed magnetic fields shows trend of decrease with Mn doping. Notably, this cause significant increase of linear magnetostriction (up to 25%) for TbFe2Mnx compounds at liquid nitrogen temperature. Thermal expansion declined due to growth of magnetic contribution in magnetically ordered state, which is linked to volume magnetostriction. These changes make TbFe2Mnx promising material for practical application. Thus, it was shown that manganese alloying opens new way for tuning magnetocrystalline anisotropy and magnetostriction in non-stoichiometric Laves phase compounds.