Millward died apparently of natural causes in Johannesburg. He had just returned from Cape Town, where he covered the Netherlands-Uruguay semifinal in his seventh World Cup.
He was found in his hotel room, and an emergency medical team was unable to revive him.
In a well-traveled AP career spanning three decades, Millward, of Birmingham, England, thrived on big events like the World Cup, the Olympics and the British Open. He joined the AP in London in 1984, left for a brief period in the late '80s and returned in 1990.
He recently counted the sports he had chronicled for the AP and came up with an astonishing total of 52.
He had been the AP's chief football writer for the past several years.
"Robert was a lovely man who truly had a zest for life," Sports Editor Terry Taylor said. "He had a twinkle in his eye, a sly sense of humor and, usually, a self-deprecating remark that made him an irresistible magnet for all of us. We loved his company as much as he loved big assignments, especially the World Cup. We will miss his good cheer and we are very, very saddened by his loss."
Millward had a deep and widespread knowledge of all international sports, including football, cricket, golf, rugby, boxing and horse racing. His main interest was football and, in particular, his beloved local team, West Bromwich Albion.
"Robert's sudden death is a blow to all his colleagues and friends at AP," International Sports Editor Simon Haydon said. "He was a gentleman among British sports writers, always ready to help and always willing and excited to cover new stories.
"Robert's enthusiasm was legendary and in South Africa he had been at his most versatile, filing impressive footballing stories until hours before his collapse. The Associated Press and sports journalism is a poorer place for Robert's death."
Millward filed his last story earlier Thursday, a preview of the World Cup final between Spain and The Netherlands. He wrote that it shaped up as a "potential classic."
Millward's wife, Pauline, said she was arranging for his body to be cremated in South Africa and his ashes returned to England. She said she would like to have them scattered at West Brom's ground, The Hawthorns.
His wife said she spoke with her husband the day before he died. Millward was due to return home this weekend and then travel to St. Andrews, Scotland, for the British Open.
"He was very happy, laughing and joking," she said. "He seemed to be enjoying himself. He was looking forward to coming home."
Before joining AP, Millward worked at the Birmingham Post. He also worked briefly in Birmingham in the late 1980s as news editor of Caters News, Britain's oldest independent press agency.