Pigeon Guillemot (Cepphus columba) populations in Prince William Sound, Alaska, USA were damaged by the 1989 Exxon Valdez oil spill, with no sign of recovery after 20 years. The continued decline was unexpected because by 2004 there was no evidence of direct exposure to residual oil. The large guillemot population nesting at the Naked Island Group in central Prince William Sound declined by 12% per year during 1990–2008 but was stable at nearby islands. Mortality rates for eggs and chicks at the Naked Island Group increased after the spill, largely due to introduced American mink (Neovison vison). Prevalence of lipid-rich fish in the diet of chicks at the Naked Island Group in 2008 was similar to post-spill levels, but lipid-rich fish abundance from aerial surveys was greater in 2008 than after the spill. Evidence suggests that, although residual oil impacted guillemots and their prey for at least a decade post-spill, mink predation gradually became the primary limiting factor at the Naked Island Group during the late 1990s and into the 2000s. The magnitude and persistence of guillemot declines at the Naked Island Group indicate that this population is unlikely to recover without management to reduce predation from non-indigenous mink.