Mangakāhia was born in Lower Waihou near Panguru in the Hokianga valley. A member of the Te Rarawa iwi, she was the daughter of Re Te Tai, an influential chief, and was educated at St Mary's Convent in Auckland.
Mangakāhia was the wife of Hāmiora Mangakāhia, who, in 1892, was elected Premier of the Kotahitanga Parliament in Hawke's Bay. The following year, Meri Mangakāhia addressed the assembly (the first woman to do so), submitting a motion in favour of women being allowed to vote for, and stand as, members of the Parliament. She noted that Māori women were landowners, and should not be barred from political representation.
She later joined the women's committee of the Kotahitanga movement, remaining involved in Māori politics and welfare movements.
She died of influenza at Panguru on 10 October 1920 according to family members. She had four children.