After training for ten months, IU made her debut in 2008 under her stage name, conceived by LOEN to mean "I and You become one through music." She performed her debut single, "Lost Child" (Korean: 미아), live for the first time on the music program M Countdown on September 18, 2008, making it her debut performance as a professional singer. IU recalled the crowd had hurled insults at her during the performance and initially felt discouraged by it, although she now considers it a beneficial experience. "Lost Child" is the lead single from her debut extended play (EP) Lost and Found, which was released on September 24, 2008. For the album, IU was awarded the "Rookie of the Month" by South Korea's Ministry of Culture, Sports and Tourism in November that same year. However, the album did not do well commercially. During an interview in 2011, IU said, "My first album failed, but I'm grateful for that. If I had become successful as soon as I made my debut, I wouldn't appreciate my staff members and the popularity that I'm enjoying now."

On April 23, 2009, IU launched her first studio album, Growing Up, with the lead promotional single, "Boo". She began her album promotion the next day, performing "Boo" on KBS2's Music Bank. The song was noted for its stark contrast in musical style to "Lost Child", which was described as a "heavy" and "dark" ballad compared to the 1980s "retro sound" of "Boo". Deemed as a "strategic" transformation, the dance choreography, stage costume and hairstyle shown in live performances were used to emphasize her youthfulness and project a "cute" image. Although it drew a favorable response from the public, IU, who was fifteen years old at the time, admitted that the image made her feel awkward. Several songs from Lost and Found were featured on her debut studio album Growing Up, including "You Know" (있잖아); a new rock-style arrangement of the song was released as the follow-up single to "Boo". Towards the end of 2009, IU released her second EP, IU...IM. She began promoting its lead single, "Marshmallow" (마쉬멜로우), on music programs from November 13, 2009. Recalling the performances, IU stated on a 2013 episode of Happy Together that she disliked having to wear the girlish costumes and hairstyle fashioned for the song promotions. The performances were well-received and were once again labelled "cute", reminiscent of reviews for "Boo".

As her popularity began to rise, IU made more frequent appearances on variety shows, performing on Star Golden Bell, Kim Jung-eun's Chocolate, and You Hee-yeol's Sketchbook. Her acoustic covers of other artists' songs such as Girls' Generation's "Gee", Super Junior's "Sorry, Sorry", and BigBang's "Lies" featured in these live performances gained tremendous interest online. In late 2009, she became a television presenter for the first time, hosting a weekly music chart show on Gom TV, while appearing as a fixed guest on multiple radio programs such as Kiss the Radio, Volume Up, MBC Standard FM's Starry Night and MBC FM4U's Best Friend Radio.