Ammassalik was one of two municipalities in Tunu, the former county of East Greenland − the other one being Illoqqortoormiut (further north).
Due to its size, the old municipality bordered more municipalities than any other in Greenland (10): For the most part, the old municipality boundaries were laid out as straight lines on the ice cap, such as the 44°W line of longitude (the western border), and 69°N circle of latitude (the northern border).
In the east, it borders on the Irminger Sea, a marginal sea of the North Atlantic Ocean, and the Denmark Strait.
Other settlements within its borders are Ikkatseq (now abandoned), Kuummiut, Kulusuk, Tineteqilaaq, Sermiligaaq and Isertoq.
Ammassalik was also the home to the Arctic Team Challenge, an adventure race.