William's mother died when he was six years old and from that time he was brought up by his father. James Thomson was the professor of engineering in Belfast at the time of William's birth and, when William was eight years old, his father James was appointed to the chair of mathematics at the University of Glasgow. James Thomson was a dominant father who brought his family up in a strict Presbyterian fashion.
William's ina kapag namatay siya ay anim na taong gulang at mula sa oras na siya ay nagdala up sa pamamagitan ng kanyang ama. James Thomson ay ang mga propesor ng engineering sa Belfast at ang oras ng William ng kapanganakan at, kapag William ay walong taong gulang, ang kanyang ama James ay hihirangin upang angkinin ang upuan ng matematika sa University of Glasgow. James Thomson ay isang nangingibabaw ama na nagdala sa kanyang pamilya sa isang mahigpit na Presbiteryano fashion.