She described herself as having "no money, no sort of influence, and no professional training of any kind, except a completely useless Arts degree".
She published two more novels, a play, two works of literary criticism—The Process of Literature and The Playgoer’s Handbook to the English Renaissance Drama—during the 1920s.
Mackenzie was a frequent contributor of reviews to The Times Literary Supplement and the New Statesman, she also lectured, and worked as a reader for publishers.
A controversial work, but well received by the reading public and reprinted, Mackenzie's study was the first serious effort which matched the popular conception of Bruce as hero, rather than the hitherto predominant academic view of Bruce as "a treacherous and rather contemptible figure" (Noble, quoting Mackenzie).
Robert Bruce, King of Scots was retrospectively subsumed into Mackenzie's six-volume history of Scotland as volume two.
She also produced a four-volume series, Scottish Pageant (1946–1950), which presented translated excerpts from documents relating to Scotland for a mass audience.