The photochemical activation of dimagnesium(I) compounds, and subsequent high yielding, regioselective reactions with inert arenes are reported. Irradiating benzene solutions of [{((Ar)Nacnac)Mg}(2)] ((Ar)Nacnac=[HC(MeCNAr)(2)](-); Ar=2,6-diisopropylphenyl (Dip) or 2,4,6-tricyclohexylphenyl (TCHP)) with blue or UV light, leads to double reduction of benzene and formation of the "Birch-like" cyclohexadienediyl bridged compounds, [{((Ar)Nacnac)Mg}(2)(mu-C6H6)]. Irradiation of [{((Dip)Nacnac)Mg}(2)] in toluene, and each of the three isomers of xylene, promoted completely regio- and chemo-selective C-H bond activations, and formation of [((Dip)Nacnac)Mg(Ar ')] (Ar '=meta-tolyl; 2,3-, 3,5- or 2,5-dimethylphenyl), and [{((Dip)Nacnac)Mg(mu-H)}(2)]. Fluorobenzene was cleanly defluorinated by photoactivated [{((Dip)Nacnac)Mg}(2)], leading to biphenyl and [{((Dip)Nacnac)Mg(mu-F)}(2)]. Computational studies suggest all reactions proceed via photochemically generated magnesium(I) radicals, which reduce the arene substrate, before C-H or C-F bond activation processes occur.