The source of the river is located in the hills near Tumbleton in the northeast of the Reach. It flows southwest past Bitterbridge and goes on to meet the Blueburn near Longtable and the Cockleswent near Cider Hall. The Mander meets another river near Highgarden and continues west, passing north of Brightwater Keep before opening onto a bay of the Sunset Sea to the east of the Shield Islands.
The river is large and slow moving, wide but with snags and sandbars that can make it hard to navigate. Beyond Bitterbridge it gets shallow, muddy and rough, and can only be navigated by small ships.
The location of Highgarden was supposedly decided by Garth the Gardener due to its proximity to the Mander river. Another King of the Reach, King Garth VII Gardener, fortified the Shield Islands and armed their smallfolk to protect the Mander from the raiders of The Iron Islands.
House Manderly once held the castle of Dunstonbury along the banks of the Mander river, but they were driven out by Lord Lorimar Peake. The heads of House Manderly claim the title Lord Marshal of the Mander to the current times.
After House Gardener's male line was extinguished in the Field of Fire the new King of Westeros, King Aegon I Targaryen, granted Highgarden to Lord Harlan Tyrell, titling him as the Lord Paramount of the Mander. The Mander supposedly ran red with blood after the Battle of Stonebridge during the Faith Militant Uprising.
During the First Blackfyre Rebellion, Ser Quentyn Ball slew all of the sons of the Lady of House Penrose at a crossing of the Mander, bar the youngest, whom he spared out of honour for the lady and her ancient house. Lord Quellon Greyjoy was killed in the Battle of the Mander during Robert's Rebellion.