The ternary phases AlT2B2 (T=Fe, Mn, Cr) and quaternary phases AlFe2–xMnxB2 have been synthesized by arc-melting and characterized by powder X-ray diffraction, magnetic measurements, Mössbauer spectroscopy, and electronic band structure calculations. All the compounds adopt the 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 magnetic measurements reveal that AlFe2B2 is a ferromagnet with TC=282K while AlMn2B2 and AlCr2B2 do not show magnetic ordering in the studied temperature range of 1.8–400K. A systematic investigation of solid solutions AlFe2−xMnxB2 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 Mössbauer spectra reveal the presence of non-magnetic (NM) and ferromagnetic (FM) spectral components in all Mn-containing samples, with the amount of NM fraction increasing as the Mn content increases. While for the AlFe2−xMnxB2 samples with x=0.0 and 0.4 the hyperfine splitting of the FM spectral component collapses at temperatures close to the Curie temperatures determined from the magnetic measurements, for the x=1.2 and 1.6 samples the FM fraction exhibits a sizable unquenched hyperfine splitting at room temperature, a finding that is inconsistent with the observed magnetic properties. Along with the increase in the amount of the NM fraction, this observation suggests formation of Fe-rich and Mn-rich regions in the structure of the solid solutions. Quantum-chemical calculations and crystal orbital Hamilton population analysis provide a clear explanation of the distinction in properties for this series of compounds and also reveal the importance of electronic factors in modifying the magnetic properties of these materials.