I91 runs along the eastern boundary shared with New Hampshire. At St. Johnsbury in northern Vermont it intersects with I93 that runs through central New Hampshire. At Lebanon in central Vermont it intersects with I89 that runs from Burlington and the capitol at Montpelier. I91 continues south to the boundary shared with Massachusetts near Brattleboro.

I91 runs mostly in the valleys between the chain of mountains. True to the name associated with Vermont, the mountain sides are green, thus, the Green Mountains as the state is sometimes called.