After no team picked him in the 1966 NFL draft, Hart's former coach Don Shroyer invited him to a tryout with the Cardinals.

[1] Hart played in relief of Terry Nofsinger in the final game of the 1966 season on December 17 for the Cardinals (who had lost Charley Johnson, who was fulfilling an ROTC commitment), and he completed four of 11 passes for 29 yards in a 38–10 loss to Cleveland.

These early career teams were mediocre at best (31–33–5 in his first seven years), but the hiring of coach Don Coryell in 1973 turned things around.

From 1974 to 1976, he guided the Cardinals to three straight ten-plus-win seasons along with back-to-back NFC East crowns in 1974 and 1975, leading the "Cardiac Cards" to ten game-winning drives during that three-year span.

Hart's early struggles would prove to doom the Cardinals, as he threw two interceptions that would each be returned for touchdowns, and the Rams would lead 28–9 at half-time, and a nascent run in the second half made the final score 35–23 in favor of Los Angeles.

Hart was named to the Pro Bowl despite the disappointment, having thrown for 2,542 yards with 13 touchdowns and 20 interceptions with a 52.4 completion percentage.

[8] He has the most passing attempts, completions, yards, touchdowns, interceptions (both career and single season), wins, and losses as a Cardinal.

[9] Hart was named the NFC Player of the Year by UPI, All-NFC and second-team All-Pro for the 1974 season.

In 1989, Hart became the athletic director for Southern Illinois University Carbondale, serving until a chancellor changeover forced him out in 1999.