John Henry Lefroy was born on January 28, 1817, in Ashe, Hampshire, England.

He was an English military man, colonial administrator, and scientist. He entered the Royal Military Academy, Woolwich in London in 1831 and became a 2nd lieutenant of the Royal Artillery in 1834. When the British government launched a project to study terrestrial magnetism, he was chosen to set up and supervise the observatory on Saint Helena. In 1842, Lefroy was sent to Toronto as the superintendent of the new Toronto Magnetic and Meteorological Observatory. He participated in the expedition to the Canadian Northwest in an attempt to map the geomagnetic activity of British North America from Montreal to the Arctic Circle, and locate the North Magnetic Pole (1843-1844). In 1848, he was made a member of the Royal Society. He remained in Toronto until 1853, continuing his observations and managing the observatory. He also helped found the Royal Canadian Institute, where he was the first vice-president in 1851-1852 and then president in 1852-1853. Before his return to London, England, he managed the transfer of the Toronto Magnetic and Meteorological Observatory to the provincial government. In London, he held various office positions in the British Army. Later, he became Inspector General of army schools and finally in 1868, director of the Ordnance Office. Lefroy was made a Companion of the Order of the Bath (CB) in 1870 and knighted in 1877 (KCMG).

In 1846, he married Emily Mary Robinson (1821–1859) and in 1860, he remarried Charlotte Anna Dundas (1824–1903). He died on April 11, 1890, in Liskeard, Cornwall, England.