Jeffery was born in Dunedin to James Jeffery and his wife Annie (née Johns). The family lived in Andersons Bay, and Jeffery was educated at Andersons Bay School, where her father taught and was headmaster.
Jeffery trained as a nurse and midwife in Dunedin, qualifying as a state registered nurse in 1910. She later became Matron at the Karitane Home, which specialised in babycare. She enlisted as a military nurse in 1914 and left New Zealand on Hospital Ship No. 1, the Maheno. Her war service was varied: she helped evacuate wounded soldiers from Gallipoli, worked in Alexandria at the New Zealand General Hospital and helped evacuate casualties from Solum and Tobruk to Alexandria. Later, she served on the French ship Valdivia and evacuated wounded from the Balkans. In September 1915, she was injured and lost her right eye. In October 1915, Jeffery was aboard the SS Marquette when it was torpedoed and sunk in the Aegean Sea. She escaped in one of the few lifeboats to be successfully launched. Jeffery also nursed at the New Zealand General Hospital in Walton-on-Thames, England.
After the war, Jeffery returned to Dunedin and nursed in the soldiers' ward at the public hospital. In 1920 she joined the Health Department and worked there as a nursing inspector until her retirement.
- Thomson, Jane (1998). Southern People: A Dictionary of Otago Southland Biography. Dunedin, New Zealand: Longacre Press. p. 253. ISBN 1 877135 11 9.
- "World War I Sources at the Hocken Collections" (PDF). University of Otago Library. January 2015. Retrieved 2 September 2016.
- National Library of New Zealand. "Papers Past | ANDERSON'S BAY. (Otago Witness, 1891-12-22)". paperspast.natlib.govt.nz. Retrieved 2 September 2016.
- National Library of New Zealand. "Papers Past | NURSES' EXAMINATION. (Evening Post, 1910-07-08)". paperspast.natlib.govt.nz. Retrieved 2 September 2016.
- "Minnie Johns Jeffery". Auckland War Memorial Museum. Retrieved 2 September 2016.
- Morris, Tessa (24 April 2013). "The fortunate survivors". Stuff.co.nz. Retrieved 2 September 2016.