He served as a counterspy for the Federal Bureau of Investigation,
after joining the Massachusetts Youth Council in 1940 and then
reporting to the FBI that it was a Communist front. For some years he
was an underground agent, reporting on the Communist Party, and in New
York City in 1949 he testified for the prosecution in a conspiracy
trial of twelve Communists, who were convicted. Philbrick made his
living as salesman, advertising director, working for nuclear
regulators in Washington, D.C., and as a lecturer.