Golden cordyceps (Cordyceps militaris) is a mushroom of the genus Cordyceps. It has been used as a food supplement for both healthy and ill people. In this study, the antileukaemic cell proliferation activities of golden cordyceps extracts were examined and compared with standard cordycepin (CDCP) in EoL-1, U937, and KG-1a cells. Wilms’ tumour 1 (WT1) protein was used as a biomarker of leukaemic cell proliferation. The cytotoxicity of the extracts on leukaemic cells was determined using the MTT assay. Their inhibitory effects on WT1 protein expression and cell cycle progression of EoL-1 cells were investigated using Western blotting and flow cytometry, respectively. Induction of KG-1a cell differentiation (using CD11b as a marker) was determined using flow cytometry. The golden cordyceps extracts exhibited cytotoxic effects on leukaemic cells with the highest IC50 value of 16.5 ± 3.9 µg/mL, while there was no effect on normal blood cells. The expression levels of WT1 protein in EoL-1 cells were decreased after treatment with the extracts. Moreover, cell cycle progression and cell proliferation were inhibited. The levels of CD11b increased slightly following the treatment. All these findings confirm the antileukaemic proliferation activity of golden cordyceps.