Mosionier was born in St. Boniface, Manitoba, to Mary Clara Pelleter Mosionier. She is the youngest of four children. Mosionier grew up living in several foster homes in Winnipeg as a ward of the Children's Aid Society. She attended St. Charles Catholic School, and Gordon Bell High School, completing Grade 11.
Mosionier met and later married Bill Culleton, and they have two children. The couple separated several times, living mainly in Toronto. There Mosionier worked as a watch inspector at a factory. She enrolled at George Brown College and later worked at Wahn, Mayer and Mith law firm, as a bookkeeper. Later the couple returned to Winnipeg, where eventually they divorced. The family then moved to Vita, Manitoba, and later Oakbank and St. Norbert.
At this time Mosionier began writing her first novel In Search Of April Raintree which was published in 1983.
In 1987 Mosionier moved to Toronto, and in 1988 she married George Moehring. As of 2014 the couple live in Winnipeg.
In 2013 she addressed a crowd on the steps of the Manitoba Legislature as part of the Idle No More movement.
Mosionier's novel, In Search of April Raintree, was published in 1983. The story included traumatic incidents from her own family life, including the suicide of two of her sisters and her own experience of being raped. A revised edition of In Search of April Raintree, was published in 1984 with the title April Raintree. The main difference between the two is that April Raintree contains a more graphic rape scene.
Mosionier's second novel, published in 2000, is titled In a Shadow of Evil. It is set in the Canadian Rockies, and follows the life of a young girl going through family trauma and foster care. The novel was inspired by Mosionier's molestation by a priest at age three.
Mosionier's first children's book Spirit of the White Bison (1985) is told from the perspective of a young white bison. It portrays a distinct part of Canadian history, the fall of bison herds and the European settlers.
Mosionier's story Christopher’s Folly (1996) tells of a young man who is forced to understand his foolishness in a dream in order to earn respect for animals, and the world around him. It teaches children to respect their surroundings.
Her next book, Unusual Friendships: A little black Cat and a Little White Rat (2002) the story of an unusual friendship between a black cat and a white rat, refers to different Métis relationships. It symbolizes the difficulties that Métis persons experience through not being purebred. The white rat attempts to teach others the jig and together the cat and the rat stick together to get respect for who they are. The story was created with a rhyme pattern.
In 2009 Mosionier wrote Come Walk With Me: A Memoir, which depicts all of her life events and she shares emotional details of the experiences that she has faced. The book has received a number of positive reviews.
Mosionier also contributed to the anthology Memoir of Manitoba, a collection of stories by native authors.