Mattel Inc. Chief Executive Margo Georgiadis is leaving the toy maker after about a year in the job, a surprising shake-up at a company searching for a strategy that can end a four-year sales slump.

Former studio executive Ynon Kreiz will succeed Ms. Georgiadis, Mattel said Thursday. When Mr. Kreiz takes over on April 26, he will be the fourth person to hold the Mattel CEO title in as many years.

The...