In a 2013 interview in The New York Times Magazine, Everman said when asked about his birth, "My birth certificate says Kodiak, but I'm pretty sure it was Ouzinkie, where my parents lived in a two-room cabin with a pet ocelot named Kia." His parents had moved to the remote Spruce Island to, as guitarist and writer Clay Tarver put it, "get back to nature", but their marriage did not survive the isolation. His mother left with Jason when he was a toddler, moved to Washington, and remarried to a former Navy serviceman; the family eventually settled in Poulsbo, across Puget Sound from Seattle.
According to Everman's half-sister, with whom he grew up, "My mother was extremely depressed, an artistic genius who was also a pill-popping alcoholic. Jason and I learned to walk on eggshells and really learned to take care of ourselves." After an incident in which he and a friend blew up a toilet with an M-80 firecracker, his grandmother put him in therapy sessions to deal with his emotional issues. Everman began playing guitar during the therapy sessions; he initially picked up one of the guitars the therapist kept around his office, and the therapist then decided to play with him, hoping it would help him open up. He went on to play in several bands during his high school years. Also, he reestablished contact with his biological father, who by that time owned a fishing boat in Alaska, and worked several seasons on the boat. Prior to joining with Nirvana, he played guitar in a local band called Stonecrow with future Nirvana drummer Chad Channing.
Everman joined Nirvana in February 1989 as a second guitarist. He is listed as being second guitarist on Nirvana's Bleach and appears on the cover, but did not actually play on any of the tracks. Nirvana founder Kurt Cobain said the credit was a token of thanks to Everman for paying a fee of $606.17 to record the album. On the 2009 remastered edition of Bleach, Everman is no longer credited but can still be seen on the front cover and he is given special thanks in the booklet.
Everman toured with Nirvana the summer of 1989 in support of Bleach. He can be heard playing guitar on Trust No-One, an unofficial release of a live performance in Boston. Cobain had broken his guitar the previous night and only provided vocals, leaving the guitar playing to Everman. Everman can also be seen in the bootleg video of Nirvana playing an "in store" performance at Rhino Records in Los Angeles on June 23, 1989. During his time with Nirvana, he could sometimes be seen using Fender guitars, generally the Fender Telecaster. Nirvana ejected Everman from the band after a short time.
A two-song Nirvana session featuring Everman on guitar is available, albeit in separate releases. A Kiss cover called "Do You Love Me?" was released on a tribute album and "Dive" was released on 2004's With the Lights Out. Both tracks were recorded at The Evergreen State College's 24-track studio in June 1989.
Everman left Nirvana in July 1989 and joined Soundgarden the following year as Hiro Yamamoto's temporary successor on bass. In April 1990, he played on the band's cover of The Beatles' "Come Together", which appeared on an EP called Loudest Love. Everman appeared in Soundgarden's Louder Than Live home video. Everman left immediately after Soundgarden completed its promotional tour for Louder Than Love in mid-1990 to play bass for the band OLD (Old Lady Drivers) and Soundgarden found Yamamoto's ultimate successor, bassist Ben Shepherd. Later, in 1993, he played guitar in Mind Funk.
In September 1994, influenced by Renaissance icon Benvenuto Cellini (who stated that a well-rounded man is an artist, warrior and philosopher), he left Mind Funk to join the United States Army, subsequently serving with the Army's 2nd Ranger Battalion and later with the Special Forces, serving tours in Afghanistan and Iraq. After completing his tour of duty with the Rangers, he took a break from the service and lived in New York City where he briefly worked as a bike messenger. He then traveled to Tibet and worked and studied in a Buddhist monastery before returning to the U.S. He reentered the Army when offered the chance to join Special Forces.
After receiving an honorable discharge in 2006, Everman went on to earn a Bachelor of Arts in philosophy from Columbia University School of General Studies on May 20, 2013. General Stanley A. McChrystal wrote a letter of recommendation for his application. In September 2010, Everman conducted an interview with Music Life Radio detailing his life. He also briefly appears in a 2010 documentary on Motörhead frontman Lemmy where he described Motorhead as "Good go-to-war music."
In July 2013, The New York Times published a portrait on Everman, written by guitarist and writer Clay Tarver. The article features interviews with Everman, his family members, former band colleagues, music industry people, and soldiers. A 2014 The Daily Beast interview mentions that Everman was invited to and attended Nirvana's Rock and Roll Hall of Fame induction, that he lives in New York, has participated in writing workshops and "still goes overseas regularly, working as a consultant for the military."