Adams was born at Lidcot, a farm at Laneast, near Launceston, Cornwall, the eldest of seven children. His parents were Thomas Adams (1788–1859), a poor tenant farmer, and his wife, Tabitha Knill Grylls (1796–1866). The family were devout Wesleyans who enjoyed music and among John's brothers, Thomas became a missionary, George a farmer, and William Grylls Adams, professor of natural philosophy and astronomy at King's College London. Tabitha was a farmer's daughter but had received a rudimentary education from John Couch, her uncle, whose small library she had inherited. John was intrigued by the astronomy books from an early age.