[2] Mascis was born in Amherst, Massachusetts, the son of a dentist,[3] and grew up in the same area together with his sister Patty and older brother Mike.

[7] At 17, Mascis joined the short-lived hardcore group Deep Wound with Lou Barlow, Scott Helland, and Charlie Nakajima in the early 1980s.

[6][8] He went on to found Dinosaur Jr. with bassist Barlow and drummer Emmett Jefferson "Patrick" Murphy (aka "Murph") in 1984, switching to guitar in the process, and they achieved national success.

[11] Megan Jasper, vice president at Sub Pop Records, characterises this period as "J had some anger, like any punk rock kid.

[6] As a side project, he was the drummer in Boston doom metal group Upsidedown Cross, who released a self-titled album on Taang!

[12][13] In 1996, he had a small part in the movie Grace of My Heart and provided a ballad and a Beach Boys-like song for the soundtrack.

[6] In April 2005 Mascis, Barlow, and Murph reformed the band for a tour celebrating the re-release of the group's first three albums.

The reunited line-up has since released five new albums: Beyond in 2007, Farm in 2009, I Bet on Sky in 2012,[6] Give a Glimpse of What Yer Not in 2016, and Sweep It Into Space in 2021.

[14] In 2006 Mascis returned to drumming with his newly formed heavy metal band Witch for their self-titled debut album.

In 2010 Mascis joined with John Petkovic and Tim Parnin of Cobra Verde and Dave Sweetapple of Witch to form Sweet Apple.

[20] In April 2014 he played with reunited Nirvana on a secret gig after the Rock and Roll Hall of Fame inductions.

[4] In 1982 Mascis became straight edge, part of a hardcore punk associated subculture whose adherents avoid drug and alcohol consumption.

This features a basswood body, C-shaped maple neck, rosewood fingerboard with 9.5" radius and 21 jumbo frets, two high output single-coil Squier Jazzmaster pickups, three-position switching and dual tone circuits, gold anodized aluminum pickguard, aged white plastic parts (knobs, switch tip, pickup covers), Adjusto-Matic™ bridge with vintage-style floating tremolo tailpiece, vintage-style tuners, chrome hardware, Vintage White finish, and J Mascis signature on the back of the large '60s-style headstock.

Due to demand and chatter on popular social media platforms, Fender re-released the model in early 2023.