"My brother was a remarkable individual on every level," Allen's sister, Jody, said in a statement. "While most knew Paul Allen as a technologist and philanthropist, for us he was a much loved brother and uncle, and an exceptional friend.
"Paul's family and friends were blessed to experience his wit, warmth, his generosity and deep concern. For all the demands on his schedule, there was always time for family and friends. At this time of loss and grief for us -- and so many others -- we are profoundly grateful for the care and concern he demonstrated every day."
Allen announced just two weeks ago that he had begun treatment on the disease. Allen was initially treated for and overcame non-Hodgkin's lymphoma in 2009.
Allen, who also owned the NBA's Portland Trail Blazers, served as the owner of the Seahawks since 1997. Under his ownership of the Seahawks, the team won Super Bowl XLVIII at the end of the 2013 season and reached three Super Bowls. The team also tallied 12 playoff appearances, nine AFC West titles and posted a 189-152-1 record.
Born in Seattle on Jan. 21, 1953, Allen's love for football was first nurtured by his father, who took him to Washington Huskies games as a child. In his teenage years, he demonstrated an almost preternatural entrepreneurial ability, launching his first programming business while still in high school, according to the Seahawks' official website.
Along with Bill Gates, he would co-found Microsoft in 1975, a company that would grow into the world's most dominant player in the professional and home computing software market. Allen purchased the team from then-owner Ken Behring after Washington state voters approved a public-private partnership to build a new stadium. Behring was threatening to move to Southern California prior to the stadium vote.
"Paul Allen was the driving force behind keeping the NFL in the Pacific Northwest," NFL commissioner Roger Goodell said in a statement. "His vision led to the construction of CenturyLink Field and the building of a team that played in three Super Bowls, winning the championship in Super Bowl XLVIII.
"The raising of the '12th Man' flag at the start of every Seahawks home game was Paul's tribute to the extraordinary fan base in the Seattle community. His passion for the game, combined with his quiet determination, led to a model organization on and off the field. He worked tirelessly alongside our medical advisers to identify new ways to make the game safer and protect our players from unnecessary risk. I personally valued Paul's advice on subjects ranging from collective bargaining to bringing technology to our game. Our league is better for Paul Allen having been a part of it and the entire NFL sends its deepest condolences to Paul's family and to the Seahawks organization."
In recent years, Allen's philanthropic efforts were widely recognized in his hometown and across the world. According to the Seahawks' website, Allen's contributions have totaled more than $2 billion. He committed $30 million in 2017 to help provide permanent housing and services for Seattle's homeless population. In addition, he helped donate toward efforts aimed at combating the 2014 Ebola epidemic in West Africa, and was awarded the Carnegie Medal of Philanthropy in 2015.
"I choose optimism, I hope to be a catalyst not only by providing financial resources but also by fostering a sense of possibility," Allen said in 2015 about his efforts to help bring positive change to the world. "Encouraging top experts to collaborate across disciplines, challenge conventional thinking and figure out ways to overcome some of the world's hardest problems."
On behalf of the many companies and institutes with which Allen was involved with and founded, Vulcan Inc. CEO Bill Hilf released the following statement:
"All of us who had the honor of working with Paul feel inexpressible loss today. He possessed a remarkable intellect and a passion to solve some of the world's most difficult problems, with the conviction that creative thinking and new approaches could make profound and lasting impact.
"Millions of people were touched by his generosity, his persistence in pursuit of a better world, and his drive to accomplish as much as he could with the time and resources at his disposal.
*"Paul's life was diverse and lived with gusto. It reflected his myriad interests in technology, music and the arts, biosciences and artificial intelligence, conservation and in the power of shared experience -- in a stadium or a neighborhood -- to transform individual lives and whole communities. *
*"Paul loved Seattle and the Pacific Northwest. The impact of Paul's efforts can be seen here at every turn. But the true impact of his vision and generosity is evident around the globe. *
"Paul thoughtfully addressed how the many institutions he founded and supported would continue after he was no longer able to lead them. This isn't the time to deal in those specifics as we focus on Paul's family. We will continue to work on furthering Paul's mission and the projects he entrusted to us. There are no changes imminent for Vulcan, the teams, the research institutes or museums.
"Today we mourn our boss, mentor and friend whose 65 years were too short -- and acknowledge the honor it has been to work alongside someone whose life transformed the world."