John "Jack" Pickersgill was born in Norfolk County, Ontario, and grew up in Manitoba. He was educated at the University of Manitoba and at Oxford University. He taught history at Wesley College, Winnipeg, for eight years. In 1937, he joined the Department of External Affairs in Ottawa. Almost immediately he was assigned to the Prime Minister's Office.
Pickersgill became King's Assistant Private Secretary in July 1938. King commented: "Pickersgill ... promises to do very well. He is very keen and has a fine sense of duty." (Diary, August 2, 1938)
Shortly after that, Arnold Heeney was appointed King's Principal Secretary and Pickersgill became his second-in-command. When Walter Turnbull was named Principal Secretary, Pickersgill continued to hold the second spot. Turnbull administered the office and arranged appointments for the Prime Minister while Pickersgill drafted letters and memoranda on policy matters and assisted the Prime Minister with his speeches. King came to have great confidence in Pickersgill and depended on him. Pickersgill soon had much more influence than his job title implied.
In 1945, Pickersgill was named Special Assistant to the Prime Minister and was officially in charge of the Prime Minister's Office. After King retired, Pickersgill continued to serve the new Prime Minister, Louis St. Laurent. Pickersgill later became a Member of Parliament and a Cabinet Minister, and in 1967 he was appointed President of the Canadian Transport Commission.
Pickersgill was one of Mackenzie King's literary executors, the others being F.A. McGregor, Norman Robertson, and Dominion Archivist Dr. W. Kaye Lamb. While these executors struggled with the question of how to handle the huge amount of documentation King had accumulated, Dr. Lamb had the main responsibility for deciding which scholars should be permitted to consult it. After his retirement in 1968, Lamb moved to British Columbia and Pickersgill took over the responsibility for granting access to the King Papers, until decisions were completed about when the various series should be opened.
Pickersgill wrote several books. The Mackenzie King Record, in four volumes, is a history of the King administration, from 1939 to 1948, based heavily on the King diary. Seeing Canada Whole: A Memoir (1994) includes an interesting account of Pickersgill's years in King's office and recollections about his colleagues there.
Library and Archives Canada holds the J.W. Pickersgill Papers (MG32-B34).