Shooter began his writing career with DC Comics. At the age of 13, he sold stories to DC that were printed in Action Comics and Adventure Comics starting in July 1966. He would write for DC for about 4 years, even after management had learned of his age.
Shooter wrote extensively for Adventure Comics during which many critics claim was the Legion heyday. Shooter's pen also touched Superman, Superboy, World's Finest and Captain Action up until he quit writing in 1970.
After taking several years off, Shooter would return to write over a dozen issues of Superboy before starting his editing career at Marvel. Shooter would return to DC in 2008 to write issues 39-49 of Legion.
Many of Shooter's stories from the 60s can be found in the Legion of Super-Heroes: Archives that began publication in 1998. Comics.org has a good breakdown of which archive contains stories written by Shooter. For quick reference, it's issues 5 thru 9 and 11.