In Hawai'i, mangrove trees are introduced species that can rapidly colonize many nearshore environments. Mangroves have been introduced on O'ahu, and Rhizophora mangle in particular has created numerous problems that have led to several mangrove removals and increased interest in long-term management of mangroves. The objective of this project was to quantify current locations of mangroves and their historical rate of expansion on O'ahu. We used the Geographic Information System (GIS) to map mangroves from digitized air photographs from six time periods: 1951–1953, 1963–1965, 1978, 1982, 1991, and 2001. We found that mangroves are still expanding at a rapid rate on O'ahu 80 yr after their introduction. Mangroves have colonized many different landforms, including tidal flats, riverbanks, fishponds, canals, protected reefs, embayments, lagoons, and other protected areas. Currently, mangroves are widely distributed and occur on all coasts except the dry leeward coast and occupy a total of 147 ha. Roughly 70% (102 ha) of all mangroves occur in Pearl Harbor.