Fassel, who was a consultant with the team in 2004, will be the new offensive coordinator, and Neuheisel will be the quarterback coach.
The hirings come a day after defensive coordinator Mike Nolan was named head coach of the San Francisco 49ers.
Ironically, Billick and Fassel, who have been close friends for years, faced off against each other in the 2000 Super Bowl, when Fassel was head coach of the New York Giants.
Fassel, 55, replaces Matt Cavanaugh, who resigned under pressure one day after the regular season ended. He was 60-54 in seven seasons with the Giants.
Neuheisel, 43, has been out of football since his firing by the University of Washington in June 2003.
He was dismissed for participating in a high-priced NCAA Tournament basketball pool.
Rex Ryan, the team's defensive line coach, was promoted to take Nolan's place.