Director Richard Hill was named interim CEO to replace Clark, who led the security-software company for three years.
Hill told analysts in a conference call that Clark "has had issues with his father being ill. And the pressure is quite high that we want to continue to deliver strong financial results and also growth on the top line for shareholders, simultaneously."
Adjusted earnings in the fiscal fourth quarter were 39 cents a share on revenue of $1.19 billion. Analysts had been calling for a profit of 39 cents a share on sales of $1.21 billion.
"Our Enterprise Security revenue was below our guidance range due to lower-than-expected bookings, which led to year-over-year reported billings declining greater than we anticipated," Hill said. "Despite this weakness, we remain confident in our Integrated Cyber Defense strategy, which has produced a strong and competitive product portfolio."
Symantec said it expects current fiscal-year revenue of $4.75 billion to $4.89 billion, and earnings between 57 cents and 73 cents a share. Analysts were calling for revenue of $4.97 billion and earnings of 56 cents a share.
First-quarter profit was forecast at 30 cents to 34 cents a share, below estimates of 40 cents.
"This is a disaster of epic proportions. The guidance combined with abrupt CEO departure will be a major gut punch to the Street. There are a lot of questions with minimal answers," said Wedbush analyst Daniel Ives, Reuters reported.
The company also said Vincent Pilette, the former financial chief of Logitech International, was named chief financial officer and would begin May 21.
The stock tumbled 15.9% to $18.64 in trading Friday.