Calvin Murray Sinclair, known by his Ojibway name Mizanay Gheezhik, meaning "the One Who Speaks of Pictures in the Sky," was born on January 24, 1951, and grew up on the former St. Peter's Indian Reserve in Manitoba. Raised by his grandparents after losing both parents at a young age, Sinclair's childhood was shaped by resilience and cultural grounding. Despite the early loss of his mother, Florence, when he was just over a year old, Sinclair excelled academically and in athletics, graduating from Selkirk Collegiate Institute as class valedictorian and Athlete of the Year in 1968.

After high school, Sinclair began studying physical education at the University of Manitoba but left to care for his ailing grandmother. In the early 1970s, Sinclair took on community leadership roles, including serving as an administrator and youth worker at the Selkirk Friendship Centre and being elected vice president of the Manitoba Metis Federation for the Interlake Region. His involvement in Indigenous community affairs set him on a path toward advocating for justice and equity for his people.