The bassist on every album credited to Jason Isbell and the 400 Unit is no longer a member of folk/rock star Isbell’s longtime backing band.

Jimbo Hart, a Muscle Shoals native, announced his departure this morning via X/Twitter.

“I’m no longer a member of the 400 Unit,” Hart wrote. “I’m grateful for all the years, all the faces I’ve played for in the course of the band’s history and proud of all we accomplished. I’m happy to be moving forward into a new chapter of life and open to all possibilities, as they may arise.”

Hart contributed bass on seven of nine albums Isbell has released since Isbell parted ways with his former band, Drive-By Truckers.

Hart’s nimble, soulful lines first appeared on the 2009 self-titled album “Jason Isbell and the 400 Unit.” The only Isbell albums he doesn’t appear on are Isbell’s 2007 solo debut “Sirens of the Ditch” and 2013 breakthrough LP “Southeastern.”

With Isbell, a North Alabama native known for songs like “24 Frames” and “If We Were Vampires,” Hart has won at least one Grammy Award. Onstage, Hart and 400 Unit drummer Chad Gable wove elastic grooves for Isbell and 400 Unit’s Sadler Vaden’s Allmans-esque guitar runs.

Hart’s first live gig with Isbell was in 2006 at a now-closed fried chicken joint in Chattanooga. However, when Isbell and the 400 Unit hit the road in support of the latest album, 2023′s “Weathervanes,” Hart was not part of the touring band.

In June, Hart announced he was stepping away – then temporarily – from Isbell’s band. ”Hey y’all, you won’t see me for the first week of this next tour,” Hart wrote at the time. “I’m taking some time to take care of my mental health and resolve some old-school traumas,” he wrote. “Thank you, @jasonisbell, for giving me the grace and space to allow this process.”

In Hart’s absence, Isbell has enlisted replacement bassists, including Anna Butterss (known for her work with the likes of Phoebe Bridgers and Bright Eyes) and Catherine Popper (whose recording credits include the likes of Willie Nelson, Jack White and Ryan Adams). For this tour, Isbell’s augmented his band’s lineup with multi-instrumentalist/backing vocalist Will Johnson, formerly of the band Centro-Matic.

In addition to Isbell, Hart’s played bass with artists such as Josh Ritter, Bahamas, Morgan Wade and Isbell’s wife Amanda Shires. He’s also contributed to solo album by 400 Unit guitarist Sadler Vaden.

The 400 Unit’s core lineup also includes drummer and Tuscumbia native Chad Gamble and keyboardist Derry deBorja.

In a June interview with AL.com about new album “Weathervanes,” Gamble discussed his rhythm-section chemistry with Hart. “Jimbo’s just the sweetest guy,” Gamble said, “and at the same time just a monster on the bass. It’s one of those things where you feel like you’ve got a brother in music and in life.”

Hart began playing bass at the age of 11, learning songs like Pink Floyd’s “Money” early on, as well as material by Pearl Jam and Metallica. Dr. Dre’s hip-hop classic “The Chronic” also played a formative role in Hart’s approach to bass.

Later as an adult, Hart lived in Austin, Texas, and played with touring blues-funk artist Papa Mali. After returning to the Shoals and before joining up with Isbell, the bassist worked with Gary Nichols, later a Grammy-winning artist with bluegrass outfit The SteelDrivers.

Hart first met Isbell back in high school at a marching band competition. Isbell played trumpet for Florence’s Rogers High and Hart played bass and tuba for Sheffield High. He’d already heard of Isbell and his guitar skills. “You cross paths when you’re kids in a small town,” Hart told me in 2017. “There was a Mexican joint [the now-closed La Fonda Mexicana] we could always go to underage and hang out and eat nachos and watch the bands. They would get us to sit in and play stuff.”

In terms of Hart’s current headspace, his recent X/Twitter posts have been upbeat. On Nov. 7, replying to a fan’s tweet asking him how he was doing, Hart replied: “I’m the best I’ve ever been, thank you!

On Nov. 10, Hart posted, “Grateful that the work I did for the ‘Weathervanes’ album has garnered three Grammy nominations. Thank you to any and everyone who voted for its nomination.” On Nov. 21, he wrote, “6 months, today! No weed and not a single thought of doing harm to myself since I put it down. Amazing how changing things really changes things. Peace and love, y’all!” On Thanksgiving, Hart posted, “I’m grateful for all of you and hope it’s a wonderful day of friends and family!”

His scrappiness remains intact, too. After announcing his departure from Isbell’s band today, a X/Twitter user asked Hart, “Are you a conservative, by chance?” Isbell is an outspoken liberal. Hart’s reply, “Hell no.”

As of publishing, Isbell hasn’t posted yet on social media regarding Hart leaving the 400 Unit. AL.com reached out to publicists who work with Isbell for a statement, and if we receive one it will be added to this story.

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