Beginning in season 4 of Smallville, Clark Kent began to team up with other super powered heroes. In the episode Run, Clark teamed up with Bart Allen AKA Impulse (though pitched as the Flash).
season 5 episodes Aqua and Cyborg introduced Aquaman and Cyborg respectively.
In season 6, the Green Arrow arrived in Smallville and began to assemble the heroes all building toward the episode Justice in which the four guests along with Clark Kent teamed up to take down Lex Luthor's kryptonite experimentation facility. This episode introduced the heroes names "Impulse," "Aquaman," etc with Clark called "Boy Scout." Chloe Sullivan provided intel using the name "Watchtower."
Season 7 added the Black Canary to the League in Siren.
And, although they were never affiliated with the League, other Leaguers have also appeared on the show including the Oreo loving Martian Manhunter, Clark's cousin Supergirl, and Zatanna Mistress of Magic.
Season 9 will likely continue to introduce more Leaguers, possibly even including the infamous Wonder Twins.
Up Next: Justice League on The Batman
The Superman/Aquaman Hour of Adventure * Super Friends * The All New Super Friends Hour * Challenge of the Super Friends * Legends of the Superheroes * The World's Greatest Super Friends * Super Friends (Shorts) * Super Friends (Lost Episodes) * Super Friends: The Legendary Super Powers Show * The Super Powers Team: Galactic Guardians * Justice League of America (TV Pilot) * Justice League Unlimited on Batman Beyond * Justice League * Justice League Unlimited * Smallville