Since his youth, Mårtensson wrote mainly science fiction short stories and novels, and a lengthy fantasy work in three volumes published in 1979–83 and in a revised edition in 1997.
His work has in style and themes been compared by Swedish critics to those of Clifford D. Simak, Theodore Sturgeon, Arabian Nights and J. R. R. Tolkien.
His first novel, Detta är verkligheten ("This is reality"), was awarded at the Paneuropean convention in Trieste in 1972, and has been translated into Danish and Czech.
As a philosopher, he published a textbook of formal logic and an introduction to the philosophy of science.
After his retirement he continued to write and published essays about science fiction and some short stories.