In August 2017, JYP Entertainment (JYPE) officially announced their new reality survival show aiming to launch a male idol debut project. More details and teasers were released in the next two months, including the show's title—Stray Kids. Prior to its premiere on October 17, JYPE released Stray Kids' first music video for a song titled "Hellevator", which was later released as a digital single. Two members, Lee Know and Felix, were initially eliminated from the group, but all nine members ended up in the final line-up.
Before the official debut of Stray Kids, the nine members were chosen as new models to endorse Jambangee Jeans in 2018, which was announced on February 13 with the brand's spring collection. On June 26, they were selected as an exclusive model of Ivy Club for the 2018 Fall semester. On July 19, they received for the first set of their CF with Minute Maid Sparkling. Five days after their first CF, they were selected to be promotional models for CGV "Youth Brand Festival" from July 24 to August 31. On September 19, Stray Kids has been chosen as the newest ambassador of Lotte Duty Free. It was announced at Lotte Duty Free's official YouTube and Instagram accounts. On October 16, Stray Kids became the new model of the Korean sportswear brand “Pro-Specs”.
Originally a nine-piece group, member Woojin left Stray Kids due to undisclosed personal reasons in October 2019. The group released their extended play Mixtape in January 2018 and officially debuted on March 25 with I Am Not.
Along with the launch of Stray Kids' official website, JYPE announced the release of the group's pre-debut extended play (EP) titled Mixtape. It contains seven tracks co-written and co-composed by the members, including "Hellevator" and other songs that they performed during the show. The EP, along with the performance video of its second track titled "Beware" (Korean: Grrr 총량의 법칙), was released on January 8, 2018, while the performance video of "Spread My Wings" (Korean: 어린 날개) was uploaded online a week later. It debuted at number 2 on Gaon's Album Chart and Billboard's World Albums chart.
On June 17, 2019, Stray Kids was chosen as the newest ambassador of Talk Talk Korea Contest in 2019, and June 18, they were appointed as an honorary ambassador by the Korean Culture and Information Service, Ministry of Culture, Sports and Tourism. In November 2020, Stray Kids was chosen to become Shopee Indonesia's brand ambassador for the 11.11 Big Sale and 12.12 Birthday Sale. On February 1, 2021, Stray Kids was chosen to become new models of the brand Clio Cosmetics. In June 2021, the group was chosen to become new models of the Japanese clothing brand Wego, and the ambassador for the Korea Pavilion of Expo 2020 Dubai. In September, the group chosen to become new models of the cosmetic brand Nacific, and the sub-unit Danceracha (Lee Know, Hyunjin, and Felix) appeared in the campaign for Italian brand Etro's unisex Earthbeat sneaker with 1-minute dance individuality video.
On March 5, JYPE announced the group's third comeback on March 25, 2019 with the release of their fourth extended play Clé 1: Miroh, in commemoration of the first anniversary of the group's debut. The group earned their first music show win on 4 April on M Countdown for the album's lead single, "Miroh." On June 19, the group released their first special album Clé 2: Yellow Wood, along with title track "Side Effects" (Korean: 부작용) between the American leg and the European leg of their first world tour.
On October 28, 2019, Woojin left the group and terminated his contract due to personal circumstances. On February 27, 2021, Hyunjin went on an official hiatus due to bullying allegations against him, and has resumed the group's scheduled activities starting July of 2021.
On June 17, Stray Kids released their first studio album titled Go Live with the title track "God's Menu", and included the Korean versions of "Top" and "Slump" and previously released singles “Gone Days" and "On Track". Go Live became the group's best-selling album, debuting atop the weekly Gaon Album Chart, and sold 243,462 copies by the end of the month to take number 5 on the monthly Gaon Album Chart. The album then received its Platinum certification from the Gaon Chart for selling more than 250,000 copies on August 2020, becoming the group's first album to do so. The lead single "God's Menu" became the group's first single to appear on the weekly Gaon Download Chart, debuting at number 144. Three months later, the group released the reissue of their first studio album titled In Life on September 14. During promotions, the group received two music show wins: once on MBC M's Show Champion and once on Mnet's M Countdown. The lead single "Back Door" was recognized by Time magazine and included it at number 8 in its list of 10 Best Songs of 2020, becoming the only song from a Korean act on the list, and being described as "an artful Frankenstein that’s as catchy as it is complex".
Before the program broadcast, Stray Kids held and broadcast the first official fan meeting, Stray Kids 1st #LoveSTAY 'SKZ-X' on February 20, 2021 via V Live, and broadcast their first Japanese fan meeting – STAYing Home Meeting – for the first anniversary of their Japanese debut on March 18. The group collaborated with Swedish producer Alesso and Chinese DJ Corsak to release a song titled "Going Dumb" for the mobile game version of PUBG on March 19. The song debuted at number 13 on Billboard's Hot Dance/Electronic Songs.
At the 2020 Mnet Asian Music Awards, it was announced that Stray Kids would be joining Ateez and The Boyz on the inaugural season of Kingdom: Legendary War, a Mnet boy group competition show. BtoB, iKon and SF9 were later confirmed as participants as well. On May 28, 2021, the group released a song for the final round of the competition titled "Wolfgang", which makes the group's first appearance on the main Gaon Digital Chart with number 138. The group won the program on June 3, awarding them their own reality show as well as a Kingdom Week special show for their comeback.
Stray Kids surprise-released the third single for their Mixtape Project titled "Mixtape: Oh" on June 26. The song debuted atop Billboard World Digital Song Sales. The group released their second studio album Noeasy on August 23. It debuted at number one at the Gaon Album Chart, and has sold over 1.1 million copies as of August 2021, making Stray Kids the first act under JYP Entertainment to sell more than a million copies for an album. The lead single "Thunderous" peaked at number 33 on Gaon Digital Chart, and number 80 on Billboard Global 200, and took six wins from music programs. The group will be release their second Japanese single "Scars / Thunderous (Japanese ver.)" on October 13.