Former “American Idol” star Syesha Mercado dropped her manager like a bad habit as soon she landed a big gig doing “The Book of Mormon” … allegedly stiffing him out of hundreds of thousands of dollars.

Mercado — who finished third on Season 7 — is being sued by former manager Fred Crawford, who picked her up as a client not too long after her stint on “Idol.”

Crawford claims Mercado agreed to fork over 20% of whatever she made in the entertainment industry. Crawford says he helped her land the “Book of Mormon” deal … for which she earned $40,000 a month.

But Crawford claims right after the ink dried Mercado cut off all communication and never paid him a cent. He’s suing for his “Mormon” cut — roughly $300,000.

Crawford has a big problem … the deal, he says, was never committed to writing.

Calls to Mercado’s rep were not returned, but we’re guessing she tells a very different story.