People.com reported Murphy's mother, Sharon, found Monjack, a Briton, unresponsive Sunday night in the Hollywood home he once shared with Brittany.
Monjack was later pronounced dead of what appeared to be natural causes, but his death remains under investigation, the Web site said.
Brittany Murphy, who starred in "Clueless," "Girl, Interrupted" and "Sin City," died in December at 32. Her death was ruled an accident due to pneumonia, an iron deficiency and multiple drug intoxication.
Murphy and Monjack married in 2007.