The long narrow basin forming the firth is part of the Great Glen Fault, which runs through its namesake, the Great Glen, and Loch Linnhe before joining with the Firth of Lorn, whence it runs further southwest across Ireland.
The firth's northeast end forms a junction with several other arms of the sea, namely Loch Linnhe, the Lynn of Lorne, Loch Etive, and the Sound of Mull. Loch Spelve and Loch Don on the Isle of Mull and Loch Feochan on the mainland are inlets of the Firth of Lorn. On the southeast side, there are also several channels and sounds in the Slate Islands.
Tidal conditions in and around the Firth of Lorn cause a number of interesting phenomena, such as the Falls of Lora at the outlet of Loch Etive, and whirlpools and standing waves in the Gulf of Corryvreckan between Scarba and Jura.