Partridge Island, or Canada's Emerald Isle, is 18 km2, 0.6 km long by 0.3 km wide, at the mouth of Saint John Harbour. The island is a 300-million-year-old volcanic ash deposit, now sparsely covered by birch, spruce, willow and alder. In 1791 New Brunswick's first lighthouse was built there to provide a safe passage into the harbour. This marine aid was later assisted by a signal station, harbour buoys, and finally, the world's first steam-operated fog alarm in 1859. In 1905 New Brunswick's first Marconi radio station was built there. The island is still an operational Coast Guard base. In 1800 the Royal Artillery constructed the first military works, and during the War of 1812 the island battery was the principal defensive position. The battery was manned during the Fenian raid in 1866 and was used as a training battery until the Great War.

During both world wars the Loyal Company of Artillery manned island defences, with the military station closing in 1947. There are several military fortifications above and below ground on the island today. In 1785 the Royal Charter of Saint John set aside the island for use as a quarantine station. In 1847, 14 892 Irish and English immigrants landed at Partridge Island, of whom 1195 died; 600 were buried in mass graves in 1847. About 1200 immigrants are buried on the island in all. The island was also home to a small fishing community and a school house operated there 1871-1948. Partridge Island has twice been designated a national historic site (fog alarm, 1925, quarantine station, 1974). The island is connected to the mainland at Negrotown Point by a rubble stone breakwater.