In 1969, Hanratty was selected in the second round of the NFL Draft by the Steelers' new head coach, Chuck Noll, and was the starting quarterback for a short time before losing the job to the Steelers' No. 1 1970 overall draft pick Terry Bradshaw. Hanratty would be the last Pittsburgh-area native to start a game at quarterback for the Steelers, until Homestead native Charlie Batch would fill in for an injured Ben Roethlisberger for two games during the team's Super Bowl-winning season in 2005. Hanratty suited up for Super Bowl IX, but did not see action.
In 1975, Hanratty played in only one regular season game, for only two plays. However, he played more in the postseason, getting into two playoff games. In the AFC Championship Game he finished the game at quarterback, taking the Steelers' last two offensive snaps, after Bradshaw was hurt. He also finished Super Bowl X at quarterback in what turned out to be his last game as a Steeler, taking the team's last four offensive snaps.
Hanratty was placed on waivers by the Steelers in September 1976 and picked up in October by the expansion Tampa Bay Buccaneers, who infamously lost every game they would play that season. As the backup quarterback to Steve Spurrier, Hanratty made a handful of appearances, and his sole start came in Week 13 against his old team, the Pittsburgh Steelers. (Hanratty would be pulled in favor of Spurrier after throwing just four passes: one was intercepted, two incomplete, and one caught for a one-yard loss. The 42-0 defeat was Hanratty's last appearance in the NFL; he would retire after the 1976 season.)
He finished his career with 2,510 passing yards, 24 touchdown passes, and 35 interceptions. He completed 38 percent of his pass attempts, which led to an overall quarterback rating of 43.0.
Following his retirement from football, Hanratty worked as a stock broker for a number of years and is now retired from his second career, spending time with his family and having coached his daughter's, Clair Hanratty, championship college softball team, the Susquehanna University River hawks from 2013-2016.
Hanratty was on hand to see Brady Quinn break his record for passing touchdowns.