In 1959 Forrestall was hired as assistant curator of the newly opened Beaverbrook Art Gallery. He was responsible for cataloguing the museum's holdings and maintaining the resulting accession records. His employment was terminated after he had two epileptic seizures while at work in early 1960. In 2006 Forrestall appeared as a witness in an ownership dispute between the Beaverbrook UK Foundation and the gallery. The Foundation claimed that a number of valuable paintings had not been given to the gallery by Lord Beaverbrook, but were only on loan. Forrestall testified that he understood from his work as assistant curator and his contact with Beaverbrook that the paintings were "all one big, great gift". His testimony was an important moment in the case, which was decided in the gallery's favour.
