This series consists of letters from Cyrus MacMillan to his brother, sister and mother during World War I, and 10 years of letters to his wife written mainly between 1925 and 1940 when he was travelling with various Federal Fisheries Commissions in the Maritimes as well as opening the Prince of Wales College in P.E.I. in 1933. Incoming correspondence includes letters from Sir Arthur Currie, Andrew MacPhail, Stephen Leacock and MacKenzie King. There are thank-you letters from recipients of speeches written for them, and congratulatory messages from various correspondents.There are some letters involving MacMillan's tenure at McGill including one from Principal Cyril James re-appointing him as Dean of Arts and Science in 1944. MacMillan, himself, wrote letters of recomendation for students.