In the two years she was involved in Operation Tabarin, a covert World War II mission to Antarctica, she identified and documented many lichen species, several of them previously unknown to science.

[6] At the island, Mackenzie participated in several short manhauling expeditions,[7] involving the manual transportation of two Nansen sleds, each carrying 700 pounds (320 kg) of food and scientific equipment.

Mackenzie was a member of the sledding team that navigated the Prince Gustav Channel along the eastern coast of the Graham Land peninsula, and later ventured eastward along the southern edge of James Ross Island, eventually rounding its easternmost point before making their way northwest back to Hope Bay.

The majority of these collections were made around the bases at Port Lockroy and Hope Bay, though she also sourced specimens from nearby excursions and during three extensive sled journeys covering approximately 800 miles (1,300 km).

Mackenzie determined the genus of every lichen specimen, discovering some previously unrecorded in the Antarctic or unique to the Graham Land region.

[3] An unfortunate professional incident was shared by a family acquaintance who was assisting in the transportation of Mackenzie, returning from having spent several weeks collecting specimens in the mountains on horseback.

As Mackenzie, the driver, and her family descended from the mountains, the shaking of the truck combined with a powerful wind caused the entire stack of collected specimens, along with their valuable annotations, to be irretrievably lost.

[1] In 1950, Mackenzie was put into contact with Erling Porsild, who hired her as a cryptogamic botanist at the National Museum of Canada in Ottawa.

She continued to collect, gathering specimens from various locations in Canada, including the Rocky Mountains, Cypress Hills (Saskatchewan), Newfoundland, Cape Breton Island (Nova Scotia), and the Ottawa area.

[1] In 1953, Mackenzie was offered the directorship of the Farlow Herbarium of Cryptogamic Botany by Harvard University, and she left Canada.

During this visit, under Operation Deep Freeze (codename for a series of United States missions to Antarctica),[7] she observed the region's biological facilities and research.

[13] Mackenzie returned to Antarctica for a third time in October 1964, where she undertook scuba diving investigations with her colleagues from France and Argentina, under a grant from the National Science Foundation and with the logistic support of the Argentine Navy.

[14] Furthermore, when reviewing Dodge's 1973 publication, Lichen Flora of the Antarctic Continent and Adjacent Islands, she dismissed it for what she described as its dubious scholarship.

The move to Canada had been a significant expense, which may have contributed to Mackenzie's decision to sell her private herbarium of 3,200 specimens of European and South American plants to the Canadian Museum of Nature.

[1][16] After separation, Mackenzie visited a specialist in New York City, who diagnosed her with "dysphonia syndrome", a disorder affecting the larynx.

[20][3] Following her transition, she found herself compelled to retire prematurely, encountering disapproval from the Farlow Herbarium, a sentiment underscored by her friend, Laurence Senelick.

[16] During the next six years, Mackenzie lost interest in her work with cryptogams, preferring to translate German botanical textbooks into English.

[1] Robert Ross, who worked with Mackenzie at the British Museum in the early 1940s, described her as "always perfectly amiable and polite and never difficult to deal with, but not inclined to chat".

[2] In the obituary written by surveyor Andrew Taylor, a colleague on Operation Tabarin, he wrote that Mackenzie was broadly admired by all for her "gentle kindness and generosity".

[16] Although her original drawings and exsiccata material of this genus were lost due to fire and mildew, she published some results in an abridged form, as well as an identification key to the species of Stereocaulon.

[9] Mackenzie published 43 papers over a 43-year period; a full listing of her scientific work is given in the obituary written by George Llano.