Cain was born in New York City in 1996, the daughter of Charles and Mary E. Cain, and has three sisters, Aine, Catherine, and Mairead.
In October 2012, she came under the coaching direction of Alberto Salazar, a decorated marathon runner and coach to Mo Farah and Galen Rupp, who won gold and silver medals respectively in the men's 10,000 meter run in the 2012 Summer Olympics.