From 1957 to 1990 Campbell worked at Merck Institute for Therapeutic Research, and from 1984 to 1990 he was a Senior Scientist and Director with Assay Research and Development. He became a US citizen in 1964. One of his discoveries while at Merck was the fungicide thiabendazole, used to treat potato blight, historically a scourge of Ireland. Thiabendazole is also used to treat trichinosis in humans.