The surprise cameo marked the first time Johnson has tackled an AC/DC track live since March 2016, when the singer, on the recommendation of doctors, was forced to quit touring as he struggled with hearing problems that threatened "total deafness."
AC/DC eventually recruited Axl Rose to fill in on the postponed Rock or Bust tour dates while Johnson recovered from the potentially career-threatening injury. "That was the darkest day of my professional life," Johnson said in April 2016 of the doctors' ultimatum to quit touring.
However, just months later, Johnson was more optimistic about his future after consulting an in-ear technology specialist. "I was really moved and amazed to be able to hear music again like I haven't heard for several years now," Johnson said in June 2016.
The appearance marked the second time Johnson has performed live in the past three months: In May, the singer joined Robert Plant and Bad Company's Paul Rodgers onstage in Oxford, England to sing "Money (That's What I Want)."
"That means a lot to me, thank you so much, thank you boys," Johnson told Muse after the Reading rendition.
It's unclear what Johnson's return means for the future of AC/DC, which also had to endure the retirement of bassist Cliff Williams; the Angus Young-led band has not yet acknowledged Johnson's "Back in Black" with Muse on social media.