Here's what BNA-online has to say about Great Horned Owl "diet". Interesting enough mink doesn't appear on the list (for what that's worth, given the fact that BNA relies almost exclusively on formal studies for its data, and no such studies may exist for Great Horned Owls in mink country ).
"Depending on abundance, takes a variety of large rodents — e.g., ground squirrels (Spermophilus spp.), Norway rats (Rattus norvegicus), wood rats (Neotoma spp.), and muskrats (Ondatra zibethica). May occasionally take squirrels (Sciurus and Tamiasciurus spp.; Packard 1954 ) and woodchucks (Marmota monax; Wink et al. 1987 ), yellow-bellied marmots (M. flaviventris) and black-tailed prairie dogs (Cynomys ludovicianus; Marti 1974 ), raccoons (Procyon lotor; Bosakowski et al. 1989d ), skunks (Mephitis and Spilogale spp.), house cats (Felis sylvestris), and even porcupines (Erethizon dorsatum; Powell 1984a ).
Small mammals often taken as prey include deer mice (Peromyscus spp.), voles (Microtus spp.), kangaroo rats (Dipodomys spp.), pocket gophers (Geomys and Thomomys spp.), and moles (Parascalops and Scalopus spp.). In Jackson Co., OK, cotton rat (Sigmodon hispidus), a key winter food, constituted up to 66% of total diet biomass ( Tyler and Jensen 1981 ). In Pennsylvania, opossums (Didelphis marsupialis) contributed most (33%) to biomass ( Wink et al. 1987 ). Species taken infrequently to commonly depending on abundance include chipmunks (Tamias and Eutamias spp.), shrews (Blarina, Sorex, Cryptotus spp.), pocket mice (Perognathus spp.), house mice (Mus musculus), rice rats (Oryzomys spp.), and occasionally bats (e.g., Tadarida spp.; Tyler and Jensen 1981 Tyler and Jensen 1981 ). Known to feed on carrion (roadkill) in winter (Smith 1974a ) during prolonged period of heavy snow cover."
I'm a great fan of minks which I encounter regularly in Reno at various of the local wetlands. Elegant little creatures and always a pleasure to,see.