An eco-friendly biocatalyst was constructed in three steps. In the first step, the tosylated starch (TsST) was synthesized by using a 4-toluenesulfonyl chloride. In the second step, the aminated starch was synthesized via the reaction of TsST with para-phenylenediamine. In the third step, the magnetic biocatalyst was fabricated by an in situ coprecipitation process from ferric and ferrous salts in the existence of aminated starch (AST). The biocatalyst was characterized by ¹H NMR, EDX, FESEM, FTIR, VSM, and TGA analyses. The magnetic aminated starch (MAST) was used as a biocatalyst for the synthesis of 4-[(indol-3-yl)-arylmethyl]-1-phenyl-3-methyl-5-pyrazolone derivatives. The various products were prepared in noteworthy yields (85–93%) in fast reaction times (35–80 min) without laborious work-up procedures. The anticancer evaluation of some 4-[(indol-3-yl)-arylmethyl]-1-phenyl-3-methyl-5-pyrazolones derivatives was studied on the survival rate of breast cancer cell lines (MCF-7) and human fibroblast cells by using an MTT assay. Additionally, recovery of the biocatalyst was studied, and results showed that the MAST was easily isolated from the reaction flask and could be recycled for up to six consecutive cycles without meaningfully falling in its efficiency.