The sub-series consists of MacKenzie's correspondence from 1924 through 1978. It also includes minutes and working papers of various organizations; printed material and ephemera are interfiled throughout if attached to correspondence. The majority of the series is fully indexed by name. This correspondence series reflects the wide-ranging nature of MacKenzie's activities and his contacts. Many of the significant figures on the Canadian scene during this period are represented.
The original arrangement of the correspondence was alphabetical by year, and this system has been maintained. Correspondence post-1962 was not organized. Instead, it has been arranged under the earlier material. Correspondence for the years 1940 44 was divided into three separate units. One unit formed part of the Main Correspondence, while the other two were kept in filing cabinets believed to have originated at the University of New Brunswick. The two separated units formed part of the main series by comparing the letters. Often correspondence with one person on a single subject was split two or even three ways. These units are now interfiled in Main Correspondence (index cards identify the original location of any given piece of correspondence).

The sub-series consists of correspondence from MacKenzie's childhood through 1978. The correspondents in this series fall into two groups: relatives and close, personal friends. Consequently, the tone of the series differs considerably from that of the Main Correspondence sub-series. MacKenzie made this distinction himself as most of these letters were kept in bundles or boxes, separate from the Main Series. While there are a few letters from the 1890s and 1900s, the central part of this series begins after 1920. The materials are primarily handwritten letters to MacKenzie, although, for later years, there is the occasional typewritten copy of MacKenzie's reply.
The correspondence is arranged alphabetically and then chronologically. Each person's name is followed by nicknames and, where applicable, their relationship to MacKenzie.
Box 241 primarily contains MacKenzie's outgoing correspondence from World War I to family and friends. Some later outgoing correspondence, mainly to MacKenzie's family, is also included in box 241. Box 242 contains a small amount of correspondence with Marge MacKenzie.