He was born as Henry Slesar in Brooklyn, New York City. His parents were Jewish immigrants from Ukraine, he had two sisters Doris and Lillian. After graduating from high school, he started his career as a copywriter.

Around 1955, he started to write short stories. While working as a copywriter, he published hundreds of short stories, including detective fiction, science fiction, criminal stories, mysteries and thrillers on Playboy, Imaginative Tales and Alfred Hitchcock's Mystery Magazine. Alfred Hitchcock hired him to write a number of the scenarios for Alfred Hitchcock Presents.

He came up with the term "coffee break," he was also the person behind McGraw-Hill's massively popular "The Man in the Chair" advertising campaign.

From 1957 to 1962, he wrote the Ruby Martinson series, and later worked on Rod Serling's Twilight Zone series. The Gray Flannel Shroud (1958), his first novel, was awarded Edgar Allan Poe Award in 1960. He penned the screenplay for the 1965 film Two on a Guillotine, which was based on one of his stories. His short story "Examination Day" was used in the 1980s Twilight Zone revival .

In 1974, he won an Emmy Award as the Head Writer of CBS Daytime's The Edge of Night (1956-1984). His term as head writer was considered lengthy in terms of head writers. During that time, he was also a head writer for the Procter and Gamble soap operas Somerset (on NBC Daytime) and Search For Tomorrow. John William Corrington replaced him on SFT. During the 1974-75 television season, he was the creator and head writer for Executive Suite, a CBS primetime serial. He wrote mainly science-fiction scripts for the CBS Radio Mystery Theater during the 1970s.

In 1983, Procter and Gamble wanted to replace him as the head writer of The Edge of Night but the ABC/ABC Daytime wanted to keep him. After his replacement as head writer by Lee Sheldon, the network named him (with Sam Hall) as the new co-head writer of its soap opera One Life to Live. He left that show after one year, and was later the head writer of the CBS Daytime serial Capitol.

In 1977, he was awarded the Edgar Award.

In 2002, he died of complications from an operation.