By Scott McLennan

Refusing to bow to conventional expectations of aging is just one more outlaw accomplishment that is part of Willie Nelson’s incredible legacy.

Outlaw Festival at Xfinity Center Mansfield, September 16. (Willie Nelson, Bob Weir, String Cheese Incident, Los Lobos, and Waylon Payne)

At the Outlaw Festival – Bob Weir and Wolf Bros, with Mickey Raphael from Willie Nelson’s band sitting in on harmonica. Photo: Sam McLennan

Willie Nelson led a rousing version of “Will the Circle Be Unbroken?” as part of the finale to his hour-long headlining set at the Outlaw Music Festival’s stop Saturday at the Xfinity Center in Mansfield. With the Grateful Dead’s Bob Weir and members of the String Cheese Incident sitting in to play and sing alongside the country-music legend, whose own band recently added Waylon Payne on guitar, a job Payne’s father Jody held for 35 years before retiring, it sure did look and feel like the circle can indeed remain unbroken.

The Outlaw Festival has become Nelson’s summer tour vehicle. He headlines every night, and the supporting cast evolves as the shows move across the country. This iteration with Weir, String Cheese Incident also included Los Lobos, which tipped the bill toward the hippie end of the outlaw scene.  In a demonstration of the unruly range of the “outlaw” identity, other stops on this roadshow have featured, at various points in the lineup, Robert Plant and Alison Krauss, John Fogerty, and the Avett Brothers.

Building to the exhilarating finale in Mansfield, the Outlaw Festival started out in the late-afternoon with a solo set by Payne, who delivered a mix of sharply rendered tunes borne from being the son of two country music artists, singer Sammi Smith being his mother.

At the Outlaw Festival — Los Lobos. Photo: Sam McLennan

Then Los Lobos, celebrating its 50th anniversary, played an outstanding set that wove in elements of the band members’ Mexican heritage as well as classic rock ’n’ roll and the blues, all launched from the starting point of the funky and angular song “Dream in Blue.”

The String Cheese Incident followed, presenting a few new songs in a set that turned mind-bending during a lengthy improv-fueled trek through the tunes “Rollover,” “Valley of the Jig,” and “Joyful Sound.” The group’s open-minded approach to blending traditional music and progressive jamming propelled the band members into cosmic terrain before wending back to groove-along melodicism. SCI closed its set with Bill Nershi, who was celebrating his 62nd birthday, singing “Colorado Bluebird Sky,” which brought the band back to its bluegrass and country beginnings.

Weir and his Wolf Bros combo preceded Nelson, crafting a performance that centered on the Dead’s psychedelia, most notably via a sprawling version of “Dark Star,” as well as looks at various facets of Weir’s songwriting, ranging from the wiry “Hell in a Bucket” to the majestic “Cassidy.”

The presence of strings and horns (the so-called Wolf Pack that has been touring with Wolf Bros since last year), with the addition of piano player Jeff Chimenti and pedal steel player Barry Sless to the core group, which began with just Jay Lane on drums and Don Was on acoustic bass, encourages guitarist and singer Weir to move in impressively creative and adventurous directions. His and the Dead’s song catalogs were treated in ways that kept this band from being dismissed as “Dead and Co. lite.”

From the beautiful violin accents applied to the folk ballad “Peggy O” to the majestic swells that the strings-and-horns quartet brought to “Playing in the Band,” Weir reimagined familiar material.

At the Outlaw Festival — String Cheese Incident. Photo: Sam McLennan

Before getting to Nelson’s fest-capping set, we need to bring up harmonica player Mickey Raphael, who has been playing with Nelson since 1973. Raphael sat in with Los Lobos for a rollicking “One Time, One Night” and the gritty blues medley of “300 Pounds of Heavenly Joy”/“Walking with Frankie Lee.” Raphael then joined Weir to revisit the Jimmy Reed hit “Big Boss Man,” a staple in the Grateful Dead’s early-’70s repertoire.

Raphael, Payne, drummer Billy English, and bass player Kevin Smith supported the 90-year-old Nelson with an intuitive rapport. The rolling and rambling nature of Nelson’s set is part of the outlaw country tradition that the star of the show practically invented and most certainly popularized. The musicians kept it stripped down, all playing acoustic instruments. And Nelson has not added a new piano following the death last year of his sister Bobbie, who, like Raphael, joined the band in 1973.

In his set, Nelson paid tribute to fellow outlaws with songs by Kris Kristofferson and Waylon Jennings. It’s such a well-worn celebratory tour of the past that some tunes — “Bloody Mary Morning” for example —  come around and you need to stop and think which outlaw wrote that one (“Bloody Mary Morning” is indeed Willie’s).

One of the more poignant moments in performance came when Payne took the lead singing his mother’s signature hit, the Kristofferson-penned “Help Me Make It Through the Night.”

As Nelson worked through his familiar concert numbers — of course, “Whiskey River” opened the set, and why shouldn’t it? — you could hear his influence on the music that has come earlier in his show. Weir’s jazzy and jagged riffing on guitar, SCI’s unabashed genre hopping, and Los Lobos’s convivial call to all to dance and revel in the music — all of this musicianship could be found in Nelson’s set.

Nelson was in good form, crooning “Always on My Mind,” belting out “Roll Me Up and Smoke Me When I Die,” and rocking to “Move It on Over.” Yes, his voice is not as strong as it was in years past, and he is seated as he plays his signature acoustic guitar, Trigger. But Nelson was galvanizing because of what he was doing on stage, not simply because of who he is and what he has done in the past.

Refusing to bow to conventional expectations of aging is just one more outlaw accomplishment that is part of Willie Nelson’s incredible legacy.

Scott McLennan covered music for the Worcester Telegram & Gazette from 1993 to 2008. He then contributed music reviews and features to the Boston Globe, Providence Journal, Portland Press Herald, and WGBH, as well as to the Arts Fuse. He also operated the NE Metal blog to provide in-depth coverage of the region’s heavy metal scene.