Henry Alexander Miers was born on May 25, 1858, in Rio de Janeiro, Brazil. He was the third son and fifth of the eight children (three of whom died in infancy). His father, Francis Charles Miers, was a civil engineer of distinction, and his grandfather, John Miers, also a professional engineer, traveled widely in South America and attained distinction as a botanist before he was elected a Fellow of the Royal Society in 1843. An elder brother, Edward John Miers, was a zoologist on the staff of the British Museum. His great-grandfather was Francis Place, an active social and political reformer. His mother was Susan Fry.