She retired in October 2015 after serving beverages to dozens of home and away players, managers and journalists at St James' Park since 1963.
The club said she had been "adored by managers, players and staff alike."
Former player and manager Alan Shearer said: "She was devoted to Newcastle, always made me smile and made the very best tea."
When Ms Cassidy retired aged 88 she was a special guest at the club's game against Norwich.
In a statement, Newcastle United said: "We are deeply saddened to learn of the passing of the club's legendary tea lady, our beloved Kath Cassidy.
"Kath loyally served the club for half a century and was adored by managers, players and staff alike. She passed away on Monday, aged 90.
"The thoughts of everyone connected with Newcastle United are with Kath's family and friends at this very sad time."
Alan Shearer Tweeted: "So sad to hear the news about Kath.
"She was devoted to Newcastle, always made me smile and made the very best tea! Will miss her."
Other tributes were posted to the club's Twitter feed.
Former player Warren Barton, who became the most expensive defender in English football when he moved to the Magpies for Â£4m in 1995, described her as "a great lady".
Former Newcastle and England manager Kevin Keegan, who spent two seasons with the club in the 1980s, said Ms Cassidy was "irreplaceable."
Tributes have been paid to Newcastle United's "legendary" tea lady Kath Cassidy, who has died at the age of 90.