Carpelan was born as Miriam Cowper in Liverpool, England, on 8 July 1882. She was one of eight children born to William Cowper and Miriam Richards. She married Carl “Harry” Carpelan and the couple emigrated to the United States in 1908. They did not have children.

Carpelan and her husband were avid horse-racing fans, and were also members of the Faith Lutheran Church in Seaside, California. They worked at the Del Monte Lodge in the city. Carpelan was widowed in 1967, later relocating to Santa Cruz.

Carpelan died on 22 June 1996 at the age of 113 years, 350 days, being survived by two nieces and a nephew and their descendants. She was the fourth-oldest person in the United States – and the sixth-oldest in the world – whose age has been validated.