Cusack's cause of death is not yet known and the Chesterfield coroner in Derbyshire say they are investigating.
Her Sheffield United teammates have been paying tributes and sharing their memories of the midfielder.
"Absolutely devastating news," wrote defender Molly Graham on X. formerly known as Twitter. "Such a great person and teammate. You will be missed hugely! Rest in peace Mads."
Goalkeeper Beth Davies said she was 'heartbroken'.
She wrote: "There aren't words to say how much you'll be missed, Mads."
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And striker Sophie Haywood wrote: "I hope you know the impact you have had on me and so many others on and off the pitch.
"A true professional and such a genuine person. You are loved by many and will be so so missed Mads. Rest easy MC8."
Cusack was the longest serving member of Sheffield United women's squad, having joined the club in 2019.
She signed a contract extension with the club in July and expressed her appreciation for Sheffield United.
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"I think everyone knows how much I love this club," she said. "This club is my home."
Cusack also served as marketing executive for the club and was getting ready to play in her sixth season with the team.
Former Sheffield United captain Billy Sharp wrote: "Such a lovely girl, enjoyed some amazing times with Maddy and the Blades."
He shared a photograph showing him and Cusack celebrating as Sheffield United secured Premier League promotion.
A Sheffield primary school also fondly remembered how the 'inspirational' Cusack had paid them a visit to encourage young girls into football earlier this year.
Ecclesall Primary School wrote: "Maddy was one of three players from Sheffield United who came to inspire girls to play football at a joint event with Sheffield Girls' in March 2023 (in the falling snow)! Our thoughts are with her family, friends and the wider Sheffield United, Sheffield United Women community. A really inspirational woman!"
A number of England Lionesses have also offered condolences to Maddy's family as they prepare to hold a minute's silence and wear black armbands before their Women's Nations League match against Scotland.
Former England captain Leah Williamson wrote: "Sending all my love and condolences to Maddy's family at this awful time!"
Beth Mead also wrote that she was 'devastated' by this news.
"Had the pleasure of playing with Maddy at youth," said the Arsenal striker. "Thoughts and love to Maddy's friends and family."
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Cusack had represented England at age-group level and the FA have stated they will pay full tribute to her in due course.
Tottenham captain Beth England also wrote: "This is devastating news. My thoughts and love go out to her family, friends and everyone at Sheffield United. Rest in peace, Maddy."