Selected in the second round of the 1967 NFL Draft out of Jackson State, Lem Barney stepped into the Detroit Lions secondary and delivered one of the most explosive rookie seasons in professional football history. On his very first NFL pass target, Barney intercepted Green Bay Packers legend Bart Starr and returned it 24 yards for a touchdown, setting the tone for a historic debut campaign. He led the entire NFL with 10 interceptions, returning three for touchdowns, while adding another score on a punt return to comfortably capture AP Defensive Rookie of the Year honors.

Barney followed up his historic rookie campaign with a dominant 1968 season, securing seven interceptions and returning a kickoff 98 yards for a touchdown against the Los Angeles Rams, which earned him his second invitation to the Pro Bowl as well as First-Team All-Pro honors. He elevated his performance in 1969 by intercepting eight passes and recording a 74-yard punt return touchdown against Minnesota, thereby securing his second consecutive First-Team All-Pro selection. In 1970, he intercepted seven passes and led the NFL with two interception returns for touchdowns — both against Bart Starr and the Green Bay Packers, including a 49-yard pick-six in the season finale that secured Detroit's qualification for the NFC playoffs. Despite opponents tending to avoid targeting his side of the field in the mid-1970s, Barney continued to demonstrate exceptional ball skills, earning consecutive Pro Bowl selections in 1972 and 1973, while maintaining his role as the defensive linchpin of the Lions' secondary.

Opposing quarterbacks quickly realized that throwing near Barney’s side was risky. With smooth hips, a quick burst, and top ball skills developed as a former college quarterback, he became the definitive shutdown cornerback of his era. Even as offenses avoided his side, Barney achieved seven Pro Bowl selections in his first eight seasons (1967–1969, 1972–1975) and was named First-Team All-Pro twice, in 1968 and 1969.

Beyond his dominance in coverage, Barney was a dangerous open-field player when he had the ball. He mainly handled punt and kickoff returns early in his career for Detroit, amassing 1,312 punt return yards, 1,274 kickoff return yards, and three touchdowns on special teams. Alongside his strong defense, Barney scored a total of seven non-offensive touchdowns, often changing field position and offering quick scoring opportunities for the Lions.

During 11 seasons in Detroit (1967–1977), Barney played 140 games, with 56 interceptions, 1,077 return yards, and seven defensive TDs. Named to the NFL 1970s All-Decade Team, his No. 20 jersey was retired in 2004 alongside Barry Sanders. He was inducted into the Pro Football Hall of Fame in 1992 and became a Pride of the Lions member in 2009.