Fafhrd is a tall (seven feet) northern barbarian; Mouser is a small, mercurial thief, once known as Mouse and a former wizard's apprentice. Fafhrd talks like a romantic, but his strong practicality usually wins through, while the cynical-sounding Mouser is prone to showing strains of sentiment at unexpected times. Both are rogues, existing within a decadent world where to be so is a requirement of survival. They spend a lot of time drinking, feasting, wenching, brawling, stealing, and gambling, and are seldom fussy about who hires their swords. But they are humane and—most of all—relish true adventure.
The characters were loosely modelled upon Leiber himself and his friend Harry Otto Fischer. Fischer initially created them in a letter to Leiber in September 1934, naming at the same time their home city of Lankhmar. In 1936, Leiber finished the first Fafhrd and Gray Mouser novella, Adept's Gambit, and began work on a second, The Tale of the Grain Ships. At the same time, Fischer was writing the beginning of The Lords of Quarmall. Adept's Gambit would not see publication until 1947, while The Lords of Quarmall would be finished by Leiber and published in 1964 and The Tale of the Grain Ships would become the prototype for "Scylla's Daughter" (1961) and, later, the novel The Swords of Lankhmar (1968). One of Leiber's motives in creating them was to have a couple of fantasy heroes closer to true human nature than the likes of Howard's Conan the Barbarian or Burroughs's Tarzan.
The stories of the two were only loosely connected until the 1960s, when Leiber organized them into a chronology and added additional material in preparation for paperback publication. Starting as young men, the two separately meet their female lovers, meet each other, and lose both their lovers in the same night, which explains both their friendship and the arrested adolescence of their lifestyles. However, in later stories, the two mature and learn leadership and eventually settle down with new female partners on the Iceland-like Rime Isle. Leiber had long contemplated continuing the series beyond this point, but died before he managed to do so.