In Lancaster the route heads eastward, crossing two branches of the Nashua River while having a short, quarter-mile concurrency with Route 70 south of Fort Devens.

It serves as the northern terminus of Route 85 before crossing into Middlesex County and the town of Stow.

It crosses through the southern tip of Concord and enters the town of Lincoln over the Sudbury River.

In Lincoln, Route 117 passes through the southwest end of town, intersecting Route 126 and the Fitchburg Line south of the Lincoln station before crossing into Weston.

In Weston the route bends southeastward through the Silver Hill, Hastings and Kendal Green sections of town before entering Waltham.