Harris was not chosen in the 1970 NFL draft, but the Cowboys invited him to training camp and he signed as a free agent. He beat out Cowboys third-round draft choice Charlie Waters (who did not crack the starting lineup until the retirement of Cornell Green following the 1974 season) for the starting free safety position his rookie year. Military service caused him to miss the second half of the season, although he returned in time for the Cowboys' victory in Super Bowl VI over the Miami Dolphins and never relinquished the position after 1971.