Around 1965, Don met local bluegrass legend (and 1970's Bluegrass Boy) Ralph Lewis. In 1967, Humphries joined Lewis' band "The Piney Mountain Boys" which featured Lewis (mandolin and vocals), Humphries (guitar and vocals), Larry Holcombe (guitar) and Arvil Freeman (fiddle). The bass duties were shared by Neal Clark and Gene Dillard. The band had four (4) different banjo players during Don's tenure and the list read like a "who's who" on the instrument; Bucky Hanks, Tom McKinney, Marc Pruett and Steve Sutton. "Ralph and Arvil taught me so much about the nuances of bluegrass music. It was sort of like getting a graduate degree in bluegrass." Humphries added; "Those times were really fun. Larry Holcombe and I were going to a bunch of festivals and everyone was just so much more accessible. Bill Monroe and Don Reno would be picking in the parking lot." Unfortunately, the only recording done by the Piney Mountain Boys was a 45 rpm with two cuts; John Fogerty's "Proud Mary" and Jake Landers' "My Last Request". When Arvil Freeman was unable to make the trip to Nashville to participate in the project, Vassar Clements filled in on fiddle.

Don really started to focus on writing bluegrass songs while playing with the Piney Mountain Boys. Around 1969, he wrote "The Ghost of Eli Renfro" which was first recorded by Rual Yarbrough. This is certainly one of Humphries' most popular songs and has also been recorded by Del McCoury, The Nashville Bluegrass Band and The Shenandoah Cut Ups. Other songs written during this period include "Quiet Grave" and "Blackbirds and Crows".