Bypassed by every franchise in the 1970 NFL Draft out of NAIA school Ouachita Baptist, the unheralded safety arrived at Dallas training camp as an underdog determined to make the roster. Cliff Harris did far more than snag a roster spot; his rare closing speed, open-field tackling, and fiery demeanor blew away the coaching staff, earning him the starting free safety job as a rookie.

In his early years, Harris balanced multiple roles, delivering strong special teams play and occasionally returning punts and kickoffs while learning Tom Landry's "Flex" defense. By 1974, he had become the key center fielder for the "Doomsday Defense." That season marked the start of a remarkable period during which Harris achieved six straight Pro Bowl appearances (1974–1979) and three First-Team All-Pro selections (1976, 1977, and 1978). These accomplishments solidified his status as one of the most hard-hitting and instinctive safeties in the NFL.

His skill in reading quarterbacks and making powerful hits over the middle gave the Dallas secondary a formidable advantage. Alongside Charlie Waters, Harris became part of a legendary safety duo in football history, leading a secondary that contributed to Dallas’s five Super Bowl appearances during the decade. He was key in winning two world championships for Big D, lifting the Lombardi Trophy in both Super Bowl VI and Super Bowl XII.

Harris took the football world by surprise when he opted to retire at the top of his game after the 1979 season, at just 31, to pursue business opportunities.

Over a ten-year career spent entirely in Dallas (1970–1979), Harris recorded 29 interceptions and 18 fumble recoveries, while earning a spot on the NFL 1970s All-Decade Team. The Cowboys enshrined him in their Ring of Honor in 2004, and years later, he received long-overdue induction into the Pro Football Hall of Fame as part of the Centennial Class of 2020.