Herein we report the mild synthesis of 15 phosphonocinnamic esters by means of a solvent-free mechanically activated Horner-Wadsworth-Emmons (HWE) olefination, in regular to excellent yields (13–99%), from tetraethyl methylenediphosphonate, an aromatic or aliphatic aldehyde and potassium tert-butoxide, using 5 min of mortar and pestle grinding. Potassium tert-butoxide acts as a base and liquid-assisting grinding agent. All isolated products were fully characterized as (E)-isomers through NMR spectroscopy. Furthermore, among the synthesized compounds diethyl (E)-2-(4-methoxyphenyl)vinylphosphonate 1 was discovered as an interesting cytotoxic agent inhibiting growth of two breast cancer cell lines (MDA-MB-453, IC₅₀ = 79 µg/mL and MCF-7 IC₅₀ = 64 µg/mL). To the best of our knowledge, this is the first report of mechanically activated HWE reaction for the synthesis of α, β-unsaturated phosphonates and the first time some of them have been assessed as cytotoxic agents on breast cancer cell lines. In conclusion, the fast and operationally simple synthesis of (E)-phosphonocinnamic esters afforded new promising cytotoxic agents in regular to excellent yields in very short reaction times.