Cathlamet is not on the way to anywhere, which is much of the point.
From Portland, Oregon: Take US-30 west along the Columbia River's south bank — a scenic two-hour drive through the Columbia River Gorge and past the industrial waterfront at Rainier. Cross into Washington via the Wahkiakum County ferry from Westport, Oregon to Puget Island (runs hourly; about 10 minutes). From Puget Island, the Julia Butler Hansen Bridge leads directly into Cathlamet. Total driving time from Portland is roughly 2 hours including the ferry.
From Longview/Kelso (Washington): Take WA-4 west along the north bank of the Columbia for about 35 miles. WA-4 runs directly into Cathlamet. This is the most direct route from anywhere on the I-5 corridor.
From Astoria, Oregon: Take US-30 east about 65 miles to Westport, then the ferry into Puget Island.
There is no public transit to Cathlamet. A car is essential.