Rainfall is higher on the southern and eastern slopes, and lower in the mountains' rain shadow to the north and west.

[3] The Nguu Mountains, along with the others in the Eastern Arc, are made up of ancient crystalline Precambrian rocks that were uplifted over millions of years along fault lines.

The long-term persistence of a humid climate and the isolation of each mountain range has led to a great deal of endemism, and a very diverse flora and fauna.

[6] The Nguu mountains mark the northernmost extent of the Eastern miombo woodlands.

[7] Pockets of lowland forest are found below 1000 meters elevation in the eastern portion of the mountains, although many have been cleared for cultivation.

[8] They have a flora similar to the humid Zanzibar–Inhambane coastal forests further east, with Milicia excelsa and Khaya anthotheca as dominant trees.