The Reds confirmed the death of their legendary Scottish forward, who won two First Division titles under Bill Shankly at Anfield in 1964 and 1966.
St John also scored the winner in the 1965 FA Cup final and represented his country more than 20 times before becoming the manager of Motherwell, his boyhood club, and later Portsmouth.
Liverpool FC wrote on Twitter: “We are deeply saddened by the passing of a true Anfield legend, Ian St John.
“The thoughts of everyone at Liverpool Football Club are with Ian’s family and friends at this sad and difficult time.”
While St John’s family released a statement on the club website. “It is with a heavy heart that we have to inform you that after a long illness we have lost a husband, father and grandfather,” the family said.
“He passed away peacefully with his family at his bedside. We would like to thank all the staff at Arrowe Park Hospital for their hard work and dedication during these very difficult times.
“The family would be grateful for privacy at this extremely sad time.”
St John spent a decade on Merseyside, following a £37,500 transfer from Motherwell in 1961, a club record fee at the time.
He would go on to score 118 goals in more than 400 appearances for the club.
Shankly had praised the signing of St John and credited him as a crucial part of the club’s success, saying: “The turning point and the beginning of the whole affair was the signing of Ian St John and Ron Yeats.”
The legendary manager also fought to sign St John despite resistance from the board, telling its members: “We can’t afford not to buy him.”