Barney was a second round draft choice in 1967. He was named 1967's defensive rookie of the year, and went on to be selected to seven Pro Bowls. He is widely considered to be one of the greatest defensive backs in Detroit Lions history. During his time in the NFL (all of which were in Detroit), Barney had 56 career interceptions, and also gained over 1000 yards returning kickoffs and punts. In Barney's first pro start, he intercepted the first pass in his direction from Green Bay's Bart Starr and ran for a touchdown. Lem Barney and the rest of the Lions defense were coached by six-time Pro Bowl defensive back Jim David. Lem credits Jim David as a mentor and teacher of the highest order. Jim David made the induction speech at the Pro Football Hall Of Fame induction ceremony for Lem. They remained great friends until David's death.
In 1999, he was ranked number 97 on The Sporting News' list of the 100 Greatest Football Players.
His number, 20, which was also worn by future Detroit star running backs Billy Sims and Barry Sanders, was retired by the Lions.
Both Barney and Lions teammate Mel Farr befriended Motown recording artist Marvin Gaye, when Gaye unsuccessfully tried out for the Lions in 1970. Barney and Farr sing background vocals on Gaye's classic 1971 song "What's Going On", for which Lem earned a Gold Record. Lem Barney also had a role in the film The Black Six and appeared as himself in Paper Lion. He is now a minister. Lem Barney performed the matrimonial services for Jim David's grandson, Sean Thornton and Laura Butler in 2009 at the Grosse Pointe War Memorial in Grosse Pointe, Michigan.
Lem Barney is an associate minister at Hope United Methodist Church in Southfield, Michigan.  He is active in metro Detroit philanthropies and also mentors inner-city youth through his involvement with the church. Barney was inducted into the Gridiron Greats Hall of Fame, 2010.