The trunkline turns north along Lincoln Avenue and then east on Main Street to run into downtown Niles.

The trunkline passes Webster and Long lakes as it crosses mixed forest and residential areas between Niles and Berrien Springs.

In downtown Berrien Springs, the trunkline then turns northwesterly along St. Joseph Avenue to pass Andrews University.

From there the highway travels northwesterly through Royalton Township through farm fields parallel to the St. Joseph River.

M-139 crosses the St. Joseph River one last time south of the interchange with I-94 at its exit 28; M-139 then passes through the community of Fair Plain and enters the eastern edge of Benton Harbor.

Before terminating at BL I-94 (Main Street) in Benton Harbor, the highway splits into a one-way pair just north of Empire Avenue.

[14][15] Plans for M-139 to be extended on a new east-west facility north of downtown Benton Harbor were halted in 1965 when the city and the state disagreed on how best to traverse a railroad;[16] the project had been canceled by February 21, 1966.

US 31,[21] but after the City of Niles announced its plan to take over maintenance of the section of East Main Street between Front Street/Bus.