Alymer ("Allie") Jernegan was the son of Captain Jared Jernegan from his first marriage. He was the half-brother of Laura and her brothers Prescott and Marcus.
Alymer was born in Edgartown, Massachusetts in 1854. His mother, Rebecca Ripley Jernegan, died when he was four. He was eight years old when his half-sister, Laura, was born in 1862.