A series of solid solutions AlFe2_xMnxB2 have been synthesized by arc-melting and characterized by powder X-ray diffraction, and magnetic measurements. All the compounds adopt the parent AlFe2B2-type structure, in which infinite zigzag chains of B atoms are connected by Fe atoms into [Fe2B2] slabs that alternate with layers of Al atoms along the b axis. The parent AlFe2B2 is a ferromagnet with Tc=282 K. A systematic investigation of solid solutions AlFe2_xMnx.B2 showed a non-linear change in the structural and magnetic behavior. The ferromagnetic ordering temperature is gradually decreased as the Mn content (x) increases. The substitution of Mn for Fe offers a convenient method for the adjustment of the ferromagnetic ordering temperature of AlFe2B2.