The midfielder, who also served as the club's marketing executive, died on Wednesday, the club confirmed on Thursday.
Cusack had been with the Bramall Lane team since January 2019 and passed the 100 appearance milestone with the club last season.
Stephen Bettis, the club's chief executive, said: "This is heartbreaking news for everyone at Bramall Lane.
"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.
"Whilst taking in the news and moving forward, the Club will offer as much support as possible to Maddy's family, friends and colleagues."
Cusack spent a year at Birmingham before ending up at the Blades and the Blues paid tribute to their former player, writing: "The Club are saddened to learn of the passing of former player Maddy Cusack. All of our thoughts are with Maddy’s family and friends at this extremely difficult time."
West Ham Women also shared their sympathy, saying on social media: "The thoughts of everyone at West Ham United are with Maddy’s family, friends and everyone at Sheffield United during this difficult time."