Hayden joined the U's initially as the Assistant Head Coach in September 2020, before taking charge of the first team in March 2021.
The former midfielder started his playing career with Crystal Palace, making over 250 appearances for the South London side, before making the switch to the east end with West Ham.
Mullins was a key part of the Hammers' side that gained promotion to the Premier Leage in 2004/05, going on to make over 200 appearances for the Upton Park side before levaing to join Portsmouth in 2009. He was part of the starting eleven that narrowly lost the 2010 FA Cup Final to Chelsea, staying at Pompey despite relegation the same year.
Further spells at Reading, Birmingham City and Notts County followed before he hung up his boots and joined Reading's academy set up before making the switch to Watford.
He took temporary charge of the Hornets on four occasions, but could not save them from relegation in 2020, leaving Vicarage Road at the end of that season.
Hayden was placed in charge of first team affairs in March 2021 following the departure of Wayne Brown.