Later career
In the 1970s, Grainger inked for both Marvel and DC Comics, including on the latter's Ghosts, Unknown Soldier and House of Mystery. He also inked two issues of the independent comics series Southern Knights, from publisher The Guild, and various issues of and backup stories in Grimjack and Dreadstar, from First Comics.
Grainger moved to Charlotte, North Carolina, sometime before spring 1969.[8] There he freelanced for the role-playing games company TSR, Inc.; his work there includes interior art for the "Endless Quest" series of gamebooks, including the Dungeons & Dragons book Under Dragon's Wing (Feb. 1984), the Hyborian Age book Conan and the Prophecy (June 1984), the Star Frontiers book Captive Planet (July 1984), and, with Jim Holloway, the Dungeons & Dragons book Knight of Illusion (June 1986).
Fantasy artist Clyde Caldwell described Grainger's later career, and his mentorship: "My dad was a printer and worked for a company called Radiator Specialty in Charlotte. Sam Grainger, an artist who did comics work for Marvel Comics, was employed by the same company. When I was a kid, I used to do drawings of superheroes and send them over to Sam for critiquing. After I graduated from college and decided that I wanted to become an illustrator, I did a little freelance work for Sam, who at that point in time had his own commercial animation studio in Charlotte. Later on, when I was working at TSR, Inc., Sam did some work for us on a freelance basis. He was a great guy. I learned a lot from him.[9]"
Grainger made guest appearances at the Charlotte comic book convention Heroes Convention. Before he succumbed to diabetes-related medical issues, Heroes Con in 1987 held a benefit auction for his medical expenses.[10][11] Grainger's last work was penciling and inking the posthumously published, eight-page Volstagg story "The Thief of Asgard" in Marvel Comics Presents #66 (1990).[12]