Greater Portland Landmarks is primarily a historical preservation advocacy organization for Portland, Maine. However, it offers library access to collections which include local 1924 tax assessment photographs, historic maps, and historic research on a variety of Portland structures. The society also operates the 1807 Portland Observatory, the only remaining historic maritime signal tower in the United States. The interpretive focus of the tower is maritime history.

The society offers a library and guided tours, three of which—the Portland Observatory, Eastern Cemetery, and Congress Street—are available specifically tailored to the needs of school groups. The observatory offers guided tours. The observatory website offers a webcam and historic photos.