Jane (later known as Jean) Robertson McKenzie was born on 19 January 1901 at Edendale, Southland, the fifth of seven children of Amy Evelyn Swale and her husband, Duncan McIntyre McKenzie, a Scottish blacksmith. From 1906 she attended Edendale School, obtaining a proficiency certificate in 1914. She then took a commercial course at Southland Technical College in Invercargill, travelling there each day by train from Edendale. During the First World War new employment opportunities opened up for young women with shorthand, typing and business skills and her first position was with the Invercargill Post Office. Both her parents died in 1917, and by 1918, aged 17, she had joined the Public Works Department. Thereafter she served in a number of centres before being transferred in 1924 to Wellington, where she was employed by the Main Highways Board.
Two years later, the Prime Minister's Department was established and Jean McKenzie began her long association with the Imperial Affairs Section (later the New Zealand Overseas Service), becoming secretary to the imperial affairs officer, Carl Berendsen. She was one of the first three appointments to the New Zealand diplomatic service and was very efficient. Berendsen was to play a significant role in furthering her career.
McKenzie was 31 years old when she first went overseas with the New Zealand delegation to the Imperial Economic Conference in Ottawa, afterwards remaining for a period in Toronto as assistant to the New Zealand trade commissioner, J. W. Collins. In 1936, when William Jordan was appointed high commissioner in London, she was transferred to the League of Nations section of his staff and attended meetings of the council and assembly of the league. Throughout the Second World War, in addition to her office duties, she devoted much time to organising clubs and facilities for New Zealand servicemen in London.
In 1941 a New Zealand Legation was established in Washington DC and Jean McKenzie was transferred there. After helping to set it up, she was promoted to second secretary. In 1943 she was posted as official secretary to Canberra, where Berendsen had been appointed high commissioner. She remained there for three years, on occasion deputising for him or his successor, James Barclay. Her next posting was to London in 1946 as a New Zealand delegate to the first session of the General Assembly of the United Nations, where she was a member of the Third and trusteeship committees. The crowning point of her career came in 1949 when she was appointed chargé d'affaires at the newly opened New Zealand Legation in Paris. Six years later she was promoted to the rank of minister, the first New Zealand woman to hold a position of such seniority in the Overseas Service.
Jean McKenzie was a warm and generous hostess. The New Zealand writer James Bertram recalled visiting in 1953 the 'handsome little New Zealand Legation' in Paris and enjoying 'an admirable lunch' during which the wine ran out. She was appointed a CBE that year and in 1956 was made a commandeuse de la Légion d'honneur. Later that year, aged 55, McKenzie retired and returned to New Zealand to live in Christchurch. She died suddenly on 1 or 2 July 1964. She had never married.
In view of her minimal educational qualifications Jean McKenzie had a remarkably successful career. She served at six overseas posts as well as at many United Nations meetings over a period of 24 years. The acting minister of external affairs, J. R. Hanan, recalled her belief that 'all work has dignity if it is well done', and noted that her record had been a truly remarkable one 'in which her determination, diligence, and a deep sense of pride in her work played a conspicuous part'.