Coming up as a musician in the UK, Elton John discovered Leon's piano playing on records Leon recorded with The Wrecking Crew, and became an instant fan. Leon and Elton would befriend each other when Leon attended Elton's first U.S. performance at the Troubadour in Los Angeles on August 25, 1970. Nearly 40 years later, Elton would find himself stumbling upon Leon's music again and being brought to tears remembering the impact that the music had had on him all those years earlier, and the friendship they had cultivated. Elton immediately reached out to Leon wanting to collaborate on a special project.

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The two would embark on the creation of The Union, a 16-track escapade that went on to reach #3 on both the Billboard 200 and Rolling Stone's list of the 30 Best Albums of 2010, making it Leon's highest charting album since 1972's Carney. 

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Leon's composition "If It Wasn't For Bad" garnered a nomination for Best Pop Collaboration with Vocals at the 53rd Grammy Awards, his first since 2002.

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The album was produced by T-Bone Burnett and featured performances by Booker T. Jones, Robert Randolph, Neil Young, and Brian Wilson. The collaboration was filmed by director Cameron Crowe, and The Union documentary debuted at the 2011 Tribeca Film Festival before being acquired by HBO, where it would air beginning in January 2012.

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The collaboration would lead to a renewed interest in Leon's career, including his nomination into the Rock n Roll Hall of Fame, where Elton would induct him.