Metallic quantum criticality is frequently discussed as a source for non-Fermi liquid behavior, but controlled theoretical treatments are scarce. Here we identify and study a novel magnetic quantum critical point in a two-dimensional antiferromagnet coupled to a three-dimensional environment of conduction electrons. Using sign-problem-free quantum Monte Carlo simulations and an effective field-theory analysis, we demonstrate that the quantum critical point is characterized by marginal Fermi liquid behavior. In particular, we compute the electrical resistivity for transport across the magnetic layer, which effectively acts like a Kondo impurity. Due to the presence of the marginal Fermi liquid excitations, the resistivity exhibits a linear decrease with temperature at criticality, in contrast to the usual quadratic decrease. Experimental realizations in Kondo heterostructures are discussed.