Marzano, a first-round draft pick of the Red Sox in 1984 who played for the Sox, Rangers and Mariners, was in his second season as an on-air personality at MLB.com.
"John was a beloved member of our team, a personable, terrific friend to all with whom he worked," said Bob Bowman, CEO of MLB Advanced Media. "He was an engaging, informed interviewer. His energy, knowledge of the game and comedic touch produced admirable results. We miss him dearly already."
MLB.com announced it will establish an internship program in memory of Marzano.
News of Marzano's death spread quickly throughout Major League Baseball on Saturday.
"All of Major League Baseball is in mourning today upon the news of the passing of John Marzano," Commissioner Bud Selig said. "John did a wonderful job in his role with Major League Baseball Advanced Media. We are greatly saddened by this unforeseen tragedy."
The Red Sox, with whom he began his professional career and made his Major League debut on July 31, 1987, also released a statement.
"Over the last few years, John covered a number of Red Sox games in his role as a television, radio and internet journalist," the statement said. "The same preparation and engaging personality that marked his time as a player in Boston carried over to his duties with the media. We express our heartfelt sympathies to John's family and friends at this very difficult time."
Marzano played in the Minors for his hometown organization, and the Phillies, like so many others, expressed genuine sorrow upon the news of his passing.
"John was an endeared member of the Philadelphia sports community who not only represented our city well as both an athlete and a broadcaster, but also had incredible passion for the games we play," Phillies president David Montgomery said in the statement. "Our deepest condolences go out to the Marzano family at this difficult time."
A South Philadelphia native born on Valentine's Day in 1963, Marzano attended Temple University and was inducted in that school's athletic Hall of Fame.
"We are deeply saddened by the loss of John Marzano," Temple director of athletics Bill Bradshaw told CSTV.com. "He was a terrific friend of Temple baseball and a dedicated player, broadcaster, father and husband. Our thoughts and prayers are with his family during this very difficult time."
As a professional, Marzano was a backup catcher with the Red Sox (1987-92), Rangers ('95) and Mariners ('96-98). He finished his Major League career with a .241 average, 11 homers and 72 RBIs in 794 at-bats over 301 games played.
The Red Sox weren't the first team to draft him. The Twins picked him in the third round in 1981 out of Central High School, but Marzano elected to go to Temple, where he starred for three years. Marzano also earned a spot as the starting catcher on the 1984 U.S. Olympic team that included future Major League stars Mark McGwire, Barry Larkin and Will Clark.
After being selected as the 14th overall pick in the '84 draft, Marzano reached the big leagues within three years, playing for Boston from 1987-1992. He was in the Cleveland and Philadelphia farm systems for two years. Then, in 1995, he appeared in two games for the Texas Rangers. In 1996, he earned a spot on Lou Piniella's Seattle roster and spent three years with the Mariners, becoming a popular member of the team and clearly touching many lives there as well.
"John was a good friend of mine and when I heard the news, it really shook me up," Mariners manager John McLaren said Saturday. "He had a way of always making you laugh and had a passion for baseball and for life. I smile just thinking about the guy and our friendship and he will be deeply missed."
Marzano worked for WIP Radio in Philadelphia and at Comcast SportsNet as a postgame analyst for Phillies games before joining MLB.com. Marzano served as co-host with Vinny Micucci of "Leading Off" on MLB.com's Baseball Channel every weekday at 9 a.m.
"Words cannot describe how sorry we are to have lost John's inspiring presence, but rest assured, his spirit and legacy will always influence us," said Dinn Mann, MLB.com's EVP of Content. "Our prayers go out to his family, his friends and his fans. John simply gave his all every time he went about doing his job and living his life, always grateful, always striving."
Marzano brought a unique perspective and keen knowledge of the game to his comments as an MLB.com analyst.
"(Ken Griffey Jr.) lifts and separates better than any hitter I have ever seen," Marzano said during a recent show, breaking down a highlight. "Having an opportunity to watch this man play for three years, I was so fortunate -- even though I was sitting on the bench watching him. To watch him and the plays he made in center field, crashing into walls, sometimes getting hurt, not worrying about himself but worrying about his team, it was amazing for me to watch that."
Clearly, in his short time with MLB.com, he made an impression not only with his on-air expertise but with his personality.
"John not only brought his baseball knowledge to us, but an energy and sense of humor that made us all feel better about the work we did," said Jim Jenks, VP/Executive Producer for MLB.com. "We may have lost a colleague and a great friend, but John Marzano always will be part of our productions."
Added senior producer Mike Siano: "John was just as great a person and friend as he was an analyst. His passion for this game was unmatched in the office each day and it was infectious. He will be missed beyond words."
Comcast SportsNet president Stephanie Smith had fond memories of Marzano as well.
"Comcast SportsNet suffered a great loss today with the sudden passing of John Marzano," Smith said in a statement. "John was a beloved member of our CSN family and he will be dearly missed.
"You knew the moment John entered the building because his infectious laugh permeated throughout our offices. His love and passion for baseball and life were a winning combination that endeared him to staff and viewers alike. John never forgot his Philadelphia roots and was thrilled to have a career talking Phillies baseball."
Indeed, Marzano brought his South Philly roots with him to every "Leading Off" show, providing analysis based on his extensive experience behind the plate along with an energetic sense of humor -- with a definite Philadelphia flavor.
As the bio of Marzano on the "Leading Off" blog said: "He is Rocky, Vince Papale, Mike Schmidt and the Liberty Bell all wrapped into one!"
Micucci wrote of his close relationship with Marzano in a post on the "Leading Off" blog on Saturday.
"John took me under his wing and treated me as if I were his own son," Micucci wrote. "He always spoke fondly of his former teammates, his loving family and his love of baseball. He repeatedly used to tell me that his father's best advice to him was: 'It is nice to be important but it is more important to be nice.' He lived every day that way and I will take it with me forever. I love him like family."
Marzano is survived by his wife Terri, daughters Dominique and Danielle, and two grandchildren.