Having moved to Harlech at a young age, he attended Ysgol Ardudwy, the local comprehensive school.

During his career, Jeffreys-Jones held visiting appointments, including: a Postdoctoral Fellowship at the Charles Warren Center for the Study of American History at Harvard (1971–72); a Stipendiary at the JFK Institut für Nordamerikastudien, Berlin, Germany; and a Canadian Commonwealth Fellowship and visiting professor at the University of Toronto.

Jeffreys-Jones began his scholarly pursuits examining the issue of violence in American industry during the Progressive Era, including the use of private detective agencies in labor disputes.

Jeffreys-Jones published an historical survey examining the development of American intelligence from the establishment of the Secret Service in the 19th century to the CIA in the 20th.

[12] His latest work, A Question of Standing, brings the history of the CIA up to 2022, making a case for the importance of analysts.