Portland Mountain sits on a ridge east of the main Lemhi Range crest in Idaho, due west of the ghost town of Gilmore. The standard route follows the peak's south ridge from a connecting saddle shared with a neighboring 10,724-foot peak.
Access follows the same approach used for the neighboring peak at 10,724 feet. From the roughly 10,380-foot saddle connecting that peak and Portland Mountain, scramble up the south ridge. The ridge is a long, tedious mix of boulders, talus, and rocky outcrops with many ups and downs; it is easy at first, steepens through a mid-section, and then flattens out for a lengthy final stretch to the high point.
The route can be descended by reversing the south ridge, or via a ridge traverse back over the neighboring peak's northeast ridge.