It runs for about 1.25 miles (2.01 km) from the town's Main Street, State Route 115, in the north to its merge with Middle Road in the south.

The Beth Condon Memorial Pathway, part of the East Coast Greenway, originates on the western side of the Portland Street and Route 1 intersection.

[3] Ammi Storer, the original owner of a business in the brick building at the corner of Main and Portland, lived at number 17.

[3] In an 1875 photograph of the northern end of Portland Street, with the Universalist church in the background, Englishman Captain Henry Newton's house (number 34, built in 1856) is visible on the right.

[3] Number 29, the circa-1849 Francis W. Seabury House, including its barn, is eligible for listing as an example of well-preserved Greek Revival design.

[3] At number 61 (near the intersection with High Street), is the 1833 Federal-style cape that was owned by Davis Moxcey, a local shipwright in the early years of shipbuilding.

Halfway along this northern section of Portland Street, at number 115, is a three-story Federal-style building that was once a tavern, built around 1810 by Colonel Seth Mitchell.

[10] Number 159, the last house on the right before the gas station, is a cape with high-style Greek Revival details, including oval windows framed with garlands set in the cornice.