The Long-tailed Thomasomys is recorded on the eastern Andean slopes of northern and central Peru, from specimens collected at 2,280–3,350 m.
Its back is a deep burnt umber, and its tail is as long as or longer than its head-and-body length; measured specimens were 100–121% of that length.
Its ecology is poorly known. Records include ground-caught specimens in humid montane forest and, in the Cordillera Carpish, disturbed forest near potato fields.
An extinction-risk assessment has not been added yet.