After leaving Ziff-Davis, the magazines' publisher, he focused on writing his own work, often under different names. He ghost-wrote several juveniles, such as The Runaway Robot (1965), based on outlines by Lester del Rey, whose name appeared on the books. He also wrote the Sherlock Holmes part of Ellery Queen's A Study In Terror (1966), in which Ellery finds a previously unknown Sherlock Holmes manuscript.
His short story "Deadly City", which appeared in the March 1953 issue of If magazine under the pseudonym Ivar Jorgensen, was made into the motion picture Target Earth. The story is about an alien invasion of Chicago and the evacuation of the city. The aliens had destroyed several Michigan towns, killing all the inhabitants, and had moved on to Illinois. The plot revolves around five characters who remain in the deserted city. They have to survive in a city devoid of people and facing annihilation by alien invaders.
His short story "The Cosmic Frame", published in the May 1955 Amazing Stories, was made into the 1957 science fiction movie Invasion of the Saucer Men and was remade, although uncredited, in 1965 as The Eye Creatures. The 1960 The Twilight Zone episode "People Are Alike All Over" was based upon his 1952 short story "Brothers Beyond the Void". His short story "Some Day They'll Give Us Guns" was filmed for the 1952 TV series The Unexpected, which was also known as Times Square Playhouse.
His short story "Beast of the Void" (currently available in Weird Science Fiction Tales: 101 Weird Scifi Stories Vol. 2, Civitas Library Classics) was published in 1956, and introduced the concept of amorphous intelligent matter in space capable of re-forming as perfect living copies of creatures from the memories of human explorers, including the protagonist's lost wife. (A similar theme was greatly expanded by Stanislaw Lem for his 1961 novel "Solaris", which was later filmed by Andrei Tarkovsky in 1972 and by Steven Soderbergh in 2002.)