It was announced in January that the former England player and manager had been diagnosed with stage four cancer, with his family revealing he was set to begin treatment.
"It is with immense sadness that we announce that Kevin Keegan has passed away at the age of 75," a statement from Keegan's family read.
"The former England player and manager had been battling cancer and was surrounded by his wife and daughters in his final moments.
"Kevin, a double Ballon d'Or winner, was a much-loved husband, father and grandfather. The family would like to thank Kevin's incredible medical team for all their support. This is a hugely difficult time and they are requesting space and privacy."
England have announced that they will pay tribute to Keegan's life before their Nations League game with Spain at Wembley in September.
Keegan was considered one of the best players of his generation and enjoyed a stellar playing career, making almost 750 club appearances, lifting the First Division trophy three times with Liverpool and winning the Ballon d'Or back-to-back in 1978 and 1979 while playing for Hamburg in the Bundesliga.
Seven years after his retirement, he took his first coaching role as manager of a struggling Newcastle team in February 1992. The Magpies were in danger of being relegated to Division Two but Keegan kept them up and then led them to the Premier League in his first full season in charge. Then, in 1996, he took Newcastle to within touching distance of their first top-tier title since 1927.
Newcastle were Premier League title favourites for most of that 1995/96 season, at one stage being 12 points clear of Manchester United. But a dramatic collapse from Keegan's side coupled with a Sir Alex Ferguson-inspired Man Utd winning 13 of their last 15, eventually saw Newcastle finish second by four points.
Late defeats at the hands of Liverpool and Nottingham Forest in the run-in saw United eventually haul them in. As the pressure mounted on a beleaguered Keegan, who was desperate to deliver the title to a passionate Newcastle fanbase, his infamous "I'd love it.." post-match interview after victory at Leeds, aimed at Sir Alex, among others, unfortunately characterised his time at St James' Park.
He would go on to manage England, for whom he had scored 21 goals in 63 games as a player, between February 1999 and October 2000 - resigning minutes after a 1-0 defeat to Germany in the last game played at the old Wembley Stadium.
His time as national boss also saw England fail to get out of the group stage at Euro 2000, despite a 1-0 win over Germany, as defeats by Portugal and Romania piled on the pressure even before that defeat at the hands of Dietmar Hamann's free-kick four months later in the first qualifier for the 2002 World Cup.
A spell at Manchester City followed, which included promotion to the Premier League in 2002 as Division One champions, but Keegan left the club in March 2005, initially announcing his retirement from football.
Keegan made an emotional return to Newcastle in 2008, but only lasted eight months following tensions with the board and owner Mike Ashley.
Newcastle's statement in full on Keegan's passing
"We are devastated to learn of the passing of Kevin Keegan, one of the most iconic, influential and deeply-loved figures in our club's history.
"Kevin was more than a legendary footballer and manager. He was the beating heart of Newcastle United for generations of supporters.
"As a player, he brought world-class talent, ambition and belief to St. James' Park, helping to transform perceptions of what Newcastle United could be.
"As a manager, he ignited our city. Kevin gave supporters hope and pride, and led a team whose style, spirit and passion captured the imagination of football fans around the world. He was so much more than an Entertainer.
"Above all that, Kevin forged an extraordinary bond with Newcastle, built on the roots of his father, Joe. He understood our people. He embraced the north east with a passion that was genuine and unwavering, and in return, he earned his rightful place in the hearts of millions.
"Kevin's impact will not be measured by appearances or results. It will live on in the memories he created with club and country, the dreams he inspired wherever he went, and the generations of supporters around the world who fell in love with football because of him.
"At this profoundly sad time, we send our love and deepest sympathy to Kevin's wife Jean, his family, friends and all who had the privilege of knowing and working alongside him.
"Today, we mourn the loss of a true giant of our history, and at the appropriate moment, we will carefully consider how we shape a lasting tribute to Kevin's remarkable life and legacy. Rest in Peace, King Kev."
'Giant of the game' - Tributes come in for Keegan
Alan Shearer: "My Hero. My manager. My friend. Sleep easy Boss."
Jamie Carragher: "RIP Kevin Keegan. One of the greatest players this country has ever produced, trophies & goals galore, double Ballon D'or winner shows he was an absolute giant of the English game. One of the most important men in making Liverpool what it is today."
Liverpool FC: "We are deeply saddened by the passing of the legendary Kevin Keegan and mourn his loss. His indomitable spirit and remarkable contribution will be forever etched in our history and his legacy will live on. Rest in peace, Kevin."
England national team: "We are incredibly saddened by the news that Kevin Keegan OBE has passed away. Kevin earned 63 international caps and scored 21 goals between 1972 and 1982, before managing the Three Lions from 1999 to 2000. Our thoughts and condolences are with Kevin's family, friends and former clubs."
Southampton FC: "We are devastated to learn of the passing of Kevin Keegan.
"Kevin signed for the club in 1980 as the holder of the Ballon d'Or and brought great joy to fans at The Dell and across the football world. Our thoughts are with his family and friends at this difficult time."
Manchester City FC: "Everyone at Manchester City is devastated to learn our former manager Kevin Keegan has passed away, aged 75.
"Kevin was a transformative manager who brought optimism and vibrancy to City during his four-year spell in charge.
"The promotion campaign in 2001/02 will never be forgotten, and Kevin then established us once again as a top Premier League side in the years that followed. Our thoughts are with Kevin's family and friends at this very difficult time."
Former Newcastle interim co-chief executive Mehrdad Ghodoussi: "Devastated to hear about Kevin Keegan. Growing up watching football, he was one of the names that defined an era, a double Ballon d'Or winner, European Cup champion, England captain, and one of the most exciting players the game has seen.
"But what he did at Newcastle was different. He turned belief into something you could feel in the stadium. King Kev wasn't just admired, he was loved, because everything he did came from the heart. Rest in peace, Kevin. Thank you for everything you gave the game."
The English Football League: "The EFL is saddened to learn of the passing of Kevin Keegan. A legend of the English game, Keegan will be remembered by generations of football fans for his impact as a player and manager. Our thoughts are with his family and friends."
Manchester United: "We are profoundly saddened by the loss of Kevin Keegan. A giant of English football, he was a cherished figure at the very highest levels of the game. Our condolences go to Kevin's family, loved ones and all those he inspired as a manager, player and person, on and off the pitch. Rest in peace, Kevin."
Former Liverpool striker David Fairclough: "Stunned. Such sad news Kevin Keegan has passed away. Absolutely devastated to hear such awful news. So many incredible memories of KK, I'm so lucky to have spent time and shared moments that have and will last a lifetime. Heartfelt condolences and love to Jean and family. YNWA."
Fulham FC: "We're incredibly sad to hear the news that our former manager Kevin Keegan has passed away. A legend of the game, Keegan led Fulham to the Football League Second Division title in 1999. He left Craven Cottage at the end of that season to focus on his duties as England manager. Our thoughts are with his friends and family at this difficult time."