Mike Post has written the music for more classic TV series than anyone else: “The Rockford Files,” “Hill Street Blues,” “The Greatest American Hero,” “Magnum, P.I.,” “The A-Team,” “L.A. Law,” “NYPD Blue,” “Law & Order” and many others over the past 50 years.

But for his new album, “Message from the Mountains & Echoes of the Delta,” he has left his day job behind, creating two original suites for bluegrass and blues bands with orchestra. And although Post doesn’t play on it (he conducted), the project takes him back to his 1960s roots as a studio guitarist. The album is slated for release on April 5.

Post has been toiling in TV for so long (scoring an estimated 7,200 hours of TV since the early 1970s) that it’s easy to forget that he once played guitar on Sonny & Cher records, won his first Grammy at 24 for arranging “Classical Gas” and became TV’s youngest-ever music director when singer Andy Williams signed him for his NBC variety show.

Post — along with his longtime partner, arranger-composer Pete Carpenter — was the first to incorporate rock ‘n’ roll into television scores, updating the sound of TV with electric guitars, synthesizers and pop rhythm sections. The new Sony Classical project, his first original concept album, draws on those decades of experience working with orchestras and soloists.

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Yet it doesn’t sound anything like his TV music. “Message from the Mountains” is a nine-movement, 29-minute bluegrass piece about the immigrant journey to America. “It’s about how strong this country is, because it’s so diverse,” Post says. “The people that have come here come from every single acre of the world. Our strength is our diversity.”

Post’s grandfather was a farmer from Ukraine, while his mother’s ancestors were rabbis from the former Alsace-Lorraine (once part of the German Empire, now part of France), so he knows a little something about immigrants and assimilation. The narration in his first movement describes the struggles of those trying to reach the U.S., then and now.

Similarly, “Echoes of the Delta” begins with a traditional blues song (Blind Willie Johnson’s “John the Revelator”) but its 16 movements (with titles like “River Walkin'” and “Highway 41”) remind us of the African American basis for the blues and the musical legacy of the brutality of slavery. “The blues is a response to horror,” Post says.

Post conceived the album in the spring of 2020, as the pandemic shut down production and he was out of work (he still scores both “Law & Order” and its spinoff “SVU” for NBC). The challenge, he knew, would be combining two kinds of music that rarely intersected, and recording them safely, sometimes remotely, at the height of COVID.

“I could make little pieces, little tunes, and teach [band members] the tunes,” he says. “But how do I open it up so that they could add a lot of interpretation? How could they jam? And then let the orchestra be influenced by what the rhythm sections are playing?”

But he decided: “If anybody was ever going to do this in an authentic way, and straddle both sides of this line, I’m probably the right guy to do it.”

With the help of his longtime associate Jon O’Hara (credited on many “SVU” episodes for orchestration and additional music), he began work. As Post told O’Hara, “We’ll call up some geniuses that have led their lives in bluegrass, we’ll bring them in one or two at a time, and we’ll record these little pieces.” O’Hara sequenced potential orchestral beds, but, as Post said: “We’ll be ready to change that orchestration based on what they play.”

He recruited veteran five-string banjo player Herb Pederson and Grammy-winning fiddler Gabe Witcher as part of his bluegrass ensemble, and Louisiana slide-guitar great Sonny Landreth, guitarist Eric Gales and veteran bassist Abe Laboriel as the core of his blues band.

After recording the basics, they “changed the orchestration to fit the explosiveness of the jamming,” Post reports. “We moved things around in an unusual way compared to sitting down and trying to write a legit piece over six months the way you would on paper.”

Finally, over two days in October 2022, Post conducted an 83-piece L.A. orchestra for the blues suite and a 66-piece orchestra for the bluegrass suite. And then, says Post, came “the great realization, that what I thought could actually happen: A musical conversation between a blues rhythm section and a bluegrass rhythm section and an orchestra. That I could work it out so that they could respond to each other.”

They are, for all intents and purposes, bluegrass and blues concertos, which may be unique in those fields. It all took over two years of writing, planning, recording and mixing.

And while the record-setting TV composer already has an Emmy (for “Murder One”) and five Grammys (including two for “Hill Streets” and one each for “Rockford Files” and “L.A. Law”), this labor of love is especially meaningful. “They killed it,” he says. “I was in tears.”

Listen to an exclusive track here: