Since joining ESPN in 2001, in addition to writing for ESPN.com, he has also hosts his own podcast on ESPN.com titled The B.S. Report, appeared as a special contributor on the television series E:60, and serves as an executive producer of ESPN's documentary project, 30 for 30. He also has written two best-selling books and worked as a writer for Jimmy Kimmel Live!

Simmons is known for his style of writing which is characterized by mixing sports knowledge and analysis, clever prose, pop culture references, his non-sports-related personal life, and for being written from the viewpoint of a passionate sports fan. Simmons also has created numerous internet memes, most notably the Ewing Theory and the Manning Face.

In 2007, he was named the 12th-most influential person in online sports by the Sports Business Journal, the highest position on the list for a non-executive.

William J. Simmons III was born on September 25, 1969, to William Simmons and Jan Corbo. Simmons was an only child and grew up in Marlboro and Brookline, Massachusetts before moving to Stamford, Connecticut to live with his mother after his parent divorced at the age of 13. His father, William Simmons, was a school administrator, and his stepmother, Molly Clark, is a doctor. He attended The Greenwich Country Day School and then attended Brunswick School in Greenwich, Connecticut for high school. In 1988, he completed a postgraduate year at Choate Rosemary Hall, a prep school located in Wallingford, Connecticut. According to Simmons, he scored a 690 on the math portion of the SAT.

Simmons then attended the College of the Holy Cross. While attending he wrote a column for Holy Cross's school paper, The Crusader, called "Ramblings" and later served as the paper's editor. He also re-started the school’s parody newspaper and started a 12-14 page, underground, handwritten, magazine about the people in his freshman hall called "The Velvet Edge." He graduated in 1992 with a B.A. in Political Science. Subsequently, while living in Brookline, Massachusetts, he studied at Boston University where he received his master's degree in print journalism two years later.

For eight years following grad school, Simmons lived in Charlestown working various jobs before eventually landing a job at ESPN. The September after grad school, Simmons started working at the Boston Herald as a high school sports reporter, mainly "answering phones... organizing food runs, [and] working on the Sunday football scores section." Three years later he got a job as a freelancer for Boston Phoenix but was broke within three months and started bartending. In 1997, unable to get a newspaper job, Simmons "badgered" Digital City Boston of AOL into giving him a column, and he started the award winning web site, BostonSportsGuy.com while working as a bartender and waiter at night. He decided to call his column "Sports Guy" since the site had a "Movie Guy."

Originally the column was only available on AOL, and Simmons forwarded the column to his friends. He began getting email from people asking if he could put them on his mailing list. For the first 18 months, Simmons would send it to about 100 people, until it became available on the web in November 1998. The website quickly built up a national following since so many friends from high school and college all e-mailed it to each other. In the winter of 2000, Simmons thought about quitting and going into real estate since there was little money in sportswriting but didn't. In 2001, his website averaged 10,000 readers and 45,000 hits per day.

Simmons gained notoriety as "The Boston Sports Guy" which earned him a job offer from ESPN in 2001 to write three guest columns. His first column was "Is Clemens the Antichrist?" which became one of the most e-mailed articles on the site that year. Simmons quickly became the most popular columnist on the site and was given his own section of ESPN.com's Page 2, which helped both himself and Page 2 gain widespread popularity. In the first sixteen months which Simmons wrote for Page 2 the viewership doubled. In late 2004 ESPN launched an online cartoon based on his columns which Simmons later called a "debacle" which he had decided to quit. Simmons writes several columns per week for his page titled "Sports Guy's World."

As a lead columnist, Simmons is one of the country's most widely read sports writers and is considered a pioneer of sportswriting on the Internet. His readership has steadily grown since he started at ESPN.com in 2001. In 2005, according to ESPN, Simmons' column averaged 500,000 unique visitors a month. According to comScore, Simmons's column had averaged 1.4 million pageviews and 460,000 unique visitors a month over the previous six months in November 2009.

In 2007, Simmons conceived the idea for 30 for 30, a series of documentaries chronicling 30 stories from the "ESPN era." Each of the documentaries detail the issues, trends, people, teams, or events that transformed the sports landscape since the sports network was founded in 1979. He wanted feature filmmakers to recount the sports stories, people, and events from the past three decades in which they took a personal interest or involvement, however great or small, and felt that said stories had not been fully explored. Simmons and his team took special interest to "stories that resonated at the time but were eventually forgotten for whatever reason." The series premiered on October 6, 2009 with "King's Ransom" directed by Peter Berg. Simmons serves as executive producer on the project.