The $2.6 billion insurance company's stock briefly climbed more than 8 percent in extended trading. The stock is up 33 percent for the year, as of Thursday's close.
Barry Zyskind, the chairman and CEO of the company, and George Karfunkel, one of its founders, together own 40.87 percent of the stock and have been pursuing a plan to take AmTrust private, a deal which Icahn said he opposes.
"You have structured your take-over to ensure that the transaction is reviewed under the lower, more deferential 'business judgment' standard, as opposed to the enhanced, more stringent 'entire fairness' standard," the investor said in a letter.
Icahn also said the company's board set April 5 as the date of record for shareholders to vote in a June 4 meeting, without letting shareholders or the market know. He called for the board to change the record date and special meeting immediately.
AmTrust did not immediately respond to CNBC requests for comment.
— CNBC's Liz Moyer contributed to this report.