It intersects with Route 169 in Lisbon center and then becomes Newent Road, as it proceeds eastward.

[1] It has an interchange with I-395 in the borough of Jewett City and intersects with Route 201 just west of the town line.

[1] In the 1920s, a loop route of New England Route 12 from south of Taftville to Jewett City via the village of Newent was a secondary state highway known as Highway 356.

At the same time, the road connecting Jewett City and the town center of Voluntown was known as Highway 321.

Route 138 was extended to the Rhode Island state line in 1938 along former SR 420, where it connected with the westward extension of Rhode Island Route 138, which had originally ended in Newport.