The weakly bound ion–molecule complex MgC2H4+ has been studied by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 218–510 nm. Mg+ is the major photofragment throughout this range, although for λ<270 nm, charge-transfer dissociation to C2H4+ is observed as a minor channel. We have identified five absorption bands of MgC2H4+. The spectral assignment is facilitated by results from ab initio calculations for the ground and low-lying excited states of MgC2H4+. Three of the bands, 1 2B2←1 2A1, 1 2B1←1 2A1, and 2 2A1←1 2A1, are based primarily in the metal-centered Mg+(3p 2P←3s 2S) atomic transition. One of the remaining bands is assigned as 2 2B2←1 2A1, a transition correlating with the a 3B1u←X 1Ag forbidden band of C2H4, with mixed charge-transfer character. The final band, 3 2A1←1 2A1, is assigned to a metal-to-ligand charge-transfer transition, enhanced by coupling with the nearby 2 2A1 state. The 1 2B2←1 2A1 band is a broad continuum, indicative of fast predissociation in the upper state. A nonadiabatic dissociation mechanism involving C=C π-bond activation by Mg+(3p) is suggested by ab initio calculations. The 1 2B1←1 2A1 band shows pronounced vibrational structure with a strong progression in the Mg+–CH4 intermolecular stretch (ν2), and weaker progressions assigned to combination bands built on the intermolecular out-of-plane wag (ν3), and a CH2–CH2 wag (ν7). The observed vibrational constants are ω2=329, x22=−2.3, ω3=439, and ω7=1024 cm−1. Measurement of the photofragment kinetic energy release determines the bond dissociation energies for the ground state [D0″(Mg+–C2H4)=0.7±0.2 eV], and for the 1 2B1 excited state, [D0′(Mg+–C2H4)=1.8±0.2 eV]. Spectroscopic constants are in good agreement with ab initio predictions.