Mihinui was born in Whakarewarewa near Rorotua, New Zealand, to a Maori mother and an Australian father. Her mother died when she was about six years old, and she and her younger brother were fostered (see: tamaiti whangai) by their maternal grandparents in Auckland and later in Whakarewarewa. She left school at 11 and in 1936 entered the traditional family occupation of guiding tourists and dignitaries around the thermal springs of Rotorua and Whakarewarewa.
She was a founder member of the Maori Women's Health League in 1937. In 1970 she became senior guide, following the death of the famous Guide Rangi. In 1982 she became public relations supervisor of New Zealand Maori Arts and Crafts.
In 1985 she was awarded an MBE and in 2002 the Distinguished Companion New Zealand Order of Merit.