Kara Zor-El burst onto the scene, changing Superman's life, by letting him know someone else survived the end of Krypton, and it happened to be his cousin!
Back in the day, comics had giant-sized issues during their regular runs, which featured reprints of original stories for fans to get filled in on the history of characters, and Supergirl was the feature of 6 of these issues....and these giants also nicely sum up Supergirl's early history in Action Comics.
80 Page Giant #G-20
This was the first Supergirl giant, featuring reprints of Supergirl's first appearance from Action Comics #252 and more, like the first appearance of Metallo, and an early appearance of Congorilla!
But, there was more in this issue...
"The Cave-Girl of Steel" by Otto Binder and Al Plastino from Action Comics #259 (December, 1959), "Supergirl's First Romance" by Jerry Siegel and Jim Mooney from Action Comics #269 (October, 1960) with the first appearance of Jerro the Mer-Boy from Lori Lemaris' Atlantis, "Supergirl's Super-Pet" (also known as Streaky the Super-Cat), who got super powers via "X-Kryptonite", in this story by Jerry Siegel and Jim Mooney from Action Comics #261 (February, 1960)...
Supergirl didn't appear on the covers of many of these early stories, but she was growing more popular by the day!
80 Page Giant #G-33
This collection starts with Action Comics #263 (April, 1960) and "Supergirl's Darkest Day" by Otto Binder and Jim Mooney, with Supergirl working to save an alien prince, Valzorr, from the prosecutor, Zoxxo, continues with Action Comics #264 (May, 1960) and "My Father, The Cop" by Jerry Siegel and Jim Mooney, in a story originally entitled "Supergirl Gets Adopted" with Linda Lee being briefly adopted by the Wilkins...
80 Page Giant G-45
This collection begins with Action Comics #278 (July, 1961) by Jerry Siegel and Jim Mooney and "The Unknown Supergirl", with Superman planning to reveal Supergirl's existence to the world, until she loses her powers, leading to Action Comics #279 (August, 1961) by Jerry Siegel and Jim Mooney and "Supergirl's Secret Enemy", who is the Kandorian scientist, Lesla-Lar, who looks like Supergirl and secretly removed Supergirl's powers (and all the while Linda Lee gets adopted by Fred and Edna Danvers, in their first appearance), leading to Supergirl being "Trapped In Kandor" when Lesla-Lar switches places with Linda, taking over as Supergirl (convincing Superman that he should reveal Supergirl's existence), all the while with Lesla plotting with Lex Luthor to destroy Superman all in this Jerry Siegel and Jim Mooney story from Action Comics #280 (September, 1961)...
All this and a feature on the milestones of Supergirl's life make this a milestone all in itself!
This reprint starts with "The Battle of the Super-Pets" from Action Comics #277 (June, 1961) by Jerry Siegel and Jim Mooney, featuring a fight between Superman's dog, Krypto, and Supergirl's cat, Streaky, Supergirl becoming "the Bride of Mr. Mxyzptlk" from Action Comics #291 (August, 1962) by Jerry Siegel and Jim Mooney, "Supergirl's Farewell To Earth" from Action Comics #258 (November, 1959) by Otto Binder and Jim Mooney with Supergirl first meeting Krypto and learning that Superman is Clark Kent...
Supergirl was growing more popular, but her run in Action Comics came to an end with Action Comics #376 (May, 1969).
This collection starts with "Superman's Super-Courtship" from Action Comics #289 (June, 1962) by Jerry Siegel and Jim Mooney, with Supergirl playing matchmaker for her cousin (in a story retitled "When Supergirl Played Cupid", and features members of the adult Legion like Saturn Woman, Triplicate Woman and Phantom Woman, as well as Supergirl look-a-like Luma Lynai), "The Secret Identity of Super-Horse" from Action Comics #301 (June, 1963) by Leo Dorfman and Jim Mooney, with Comet gaining a new mystic ability, to become human when a comet passes through Earth's atmosphere (and he assumes the identity of Bronco Bill to romance Supergirl)...
An odd collection of stories, taken a little out of order, but a great theme issue.
Super DC Giant #S-24
This collection of Supergirl stories is chronological, featuring four tales in a row, starting with Action Comics #295 (December, 1962) and "The Girl With The X-Ray Mind" by Leo Dorfman and Jim Mooney, featuring Lena Thorul (really Lex Luthor's sister, Lena Luthor), and her new FBI career (as well as ESP powers), leading to Action Comics #296 (January, 1963) with "The Girl Who Was Supergirl's Double" by Leo Dorfman and Jim Mooney, with Lena wearing a Supergirl costume to a party, and Dick Malverne thinking Lena is Supergirl...
Phantom Zone villains, but paying the price when Kru-El disintegrates Lesla-Lar, and Action Comics #298 (March, 1963) by Leo Dorfman and Jim Mooney, and "The Super-Powers of Lex Luthor" with Supergirl enlisting the help of Luthor against the Phantom Zone villains (until they give Lex Kryptonian powers).
All this and "new Supergirl costumes designed by YOU, the readers!" make this a truly SUPER giant!
DC 100 Page Super Spectacular #DC-10
Golden Age Black Canary from Flash Comics #86 (August, 1947), Wonder Woman's Golden Age battle against Villainy Incorporated from Wonder Woman #28 (March-April, 1948), a Phantom Lady story from Police Comics #17 (March, 1943) and the last Golden Age story of Merry the Girl of 1,000 Gimmicks from Star-Spangled Comics #90 (March, 1949), along with a guide to the females on the cover of the issue (Hawkgirl, Thorn, Platinum, Zatanna, Big Barda, Beautiful Dreamer, Star Sapphire, Liberty Belle, Lilith, Wonder Girl, Merry, the Enchantress, Dumb Bunny, Phantom Lady, the Cheetah, the Harlequin, Batgirl, Black Canary and Wonder Woman!).