The singer said: "He's obviously intelligent. He's got a big side of him that's interested in psychology. You go into his office and there are books on psychology.
"There's a lot more to him than what the public sees. He's a very charming man and very interesting. You can't deny that he has something very special."
Mark added that Cowell, 50, had taken a close personal interest in Westlife's new album.
The band, who are signed to the pop mogul's label Syco Music and managed by Louis Walsh, release Where We Are tomorrow. And Mark, 29, said Cowell had given his seal of approval to the record.
He said: "Simon sent a text saying, 'Oh my God, guys, I have to tell you, hats off, your album is amazing.' I was so happy."
Mark also said Cowell had tried to persuade Westlife to cover Michael Jackson's You Are Not Alone before The X Factor finalists.
He laughed: "If you heard some of the cover songs we turned down, like You Are Not Alone. Now it's the X Factor single, so Simon's convinced someone to do it, and it wasn't us!"
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