Haslett attended Indiana University of Pennsylvania, and was selected in the second round of the 1979 NFL draft by the Buffalo Bills.

[7] That season, he guided the Saints to a 10–6 regular-season record, their second NFC West division championship, and the first playoff victory in franchise history (defeating the St. Louis Rams; they lost to the Minnesota Vikings the next week).

As a result of the Saints' turnaround from their previous 3–13 season (under his predecessor, Mike Ditka), Haslett was named NFL Coach of the Year.

That year, the Saints started 9–4, but three consecutive losses, including to the 1-14 Cincinnati Bengals, resulted in them missing the playoffs by a single game.

On September 29, 2008, Haslett was named the interim head coach of the Rams after Scott Linehan was fired.

He won his first game as interim head coach of the Rams with a 19–17 victory over the 4-1 Washington Redskins.

On January 15, 2009, the Rams announced that Haslett was no longer in consideration for the permanent head coaching position and that the team would be going in a "new direction".

[11] Haslett was hired as the Washington Redskins defensive coordinator on January 12, 2010, under head coach Mike Shanahan.

The move reunited the pair, as Gruden had worked as the offensive coordinator under Haslett for the Tuskers in the 2009 season.

[13] After spending the 2015 season as a consultant for Penn State,[14] Haslett was hired as the linebackers coach for the Cincinnati Bengals on January 15, 2016.

[18] Haslett was announced as an XFL head coach in 2021, later revealed to be for the Seattle Sea Dragons.

Haslett was the head coach of the Sea Dragons for their final season in 2023, guiding them to a 7–3 record before losing in the divisional round of the playoffs 37–21 to the DC Defenders.