At Mother Jones, composer Jeremy Samuel Faust analyzes the theme from Mad Men that was actually snipped from RJD2's "A Beautiful Mine" from his 2006 album Magnificent City Instrumentals. (Apparently, creator Matthew Weiner originally asked Beck to write the theme.)
From Mother Jones:
Beck declined, though, betting that a show about 1960s ad executives would be a bore. Weiner later chose RJD2's "A Beautiful Mine" after he stumbled on the song while listening to public radio. I suspect that Weiner wanted from Beck something similar to what he ended up with: a delicious collage of pop postmodernity. And while the RJD2's music wasn't created for Mad Men, it was scrupulously cut from its original length of 5 minutes, 29 seconds to just 37 seconds. It has a "big old movie quality to it, and updated beat to it, it had drama," Weiner has said. "I just loved it."
"What the "Mad Men" Theme Music Has Been Trying to Tell Us All Along"