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 lambda <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(2)B(2)<--1(2)A(1), 1(2)B(1)<--1(2)A(1), and 2(2)A(1)<--1(2)A(1), are based primarily in the metal-centered Mg+(3p P-2<--3s S-2) atomic transition. One of the remaining bands is assigned as 2(2)B(2)<--1(2)A(1), a transition correlating with the a B-3(1u)<--X(1)A(g) forbidden band of C2H4, with mixed charge-transfer character. The final band, 3(2)A(1)<--1(2)A(1), is assigned to a metal-to-ligand charge-transfer transition, enhanced by coupling with the nearby 2(2)A(1) state. The 1(2)B(2)<--1(2)A(1) band is a broad continuum, indicative of fast predissociation in the upper state. A nonadiabatic dissociation mechanism involving C=C pi-bond activation by Mg+(3p) is suggested by ab initio calculations. The 1(2)B(1)<--1(2)A(1) band shows pronounced vibrational structure with a strong progression in the Mg+-CH4 intermolecular stretch (nu(2)), and weaker progressions assigned to combination bands built on the intermolecular out-of-plane wag (nu(3)), and a CH2-CH2 wag (nu(7)). The observed vibrational constants are omega(2) = 329, x(22)=-2.3, omega(3)=439, and omega(7)=1024 cm(-1). Measurement of the photofragment kinetic energy release determines the bond dissociation energies for the ground state [D-0(")(Mg+-C2H4)=0.7+/-0.2 eV], and for the 1(2)B(1) excited state, [D-0'(Mg+-C2H4)=1.8+/-0.2 eV]. Spectroscopic constants are in good agreement with ab initio predictions.