Cusack reached 100 appearances for the club last season, after joining in 2019. She also worked in the club's offices at Bramall Lane as a marketing executive.
"This is heartbreaking news for everyone," the club's chief executive officer Stephen Bettis said.
"Maddy had a unique position of being part of a number of teams at Sheffield United and was popular with everyone that she came into contact with.
"Her personality and professionalism made her a credit to her family -- she will be sadly missed."
Cusack had signed a contract extension in July for a sixth season with the club.
The club did not give any details about the cause of death.