He dropped out of school when he decided to become a singer. During his school days, he played bass guitar as a member of a band. During the years of 2001–2 when he was 14–15, whilst working as a waiter in a family restaurant, located at Jamsil-dong, he was introduced to a new set up management company CEO. Initially he was to debut in a 5 member group. Han Yeon of B2Y (now disbanded) was to be the Leader and Kim the maknae. While the group underwent training together, they were asked by their company to debut in China as a Hallyu project group, but the group rejected the offer and disbanded. Kim proceeded to audition for DSP Media (formerly DSP Entertainment) and became a member and leader of SS501. Kim went back to school and graduated in 2006 after debuting as SS501. In 2011, he enrolled in Chungwoon University to study stage production management.
On 8 June 2005, Kim made his entertainment debut as the leader of SS501. The group appeared in the 2006 show, Thank You For Waking Me Up where he was proven to be a heavy sleeper, and the MNet show Idol World in 2007, in which SS501 did various activities in Japan.
In May 2008, Kim joined the cast of We Got Married in episode nine. Known as the "Lettuce Couple", Hyun Joong and Hwang Bo went on to win MBC's Best Couple Award. The couple made their official exit from the show in episode 38, on 14 December 2008, due to scheduling conflicts with Boys Over Flowers.
2009: Acting, Boys Over Flowers and H1N1
Whilst in Japan, in 2008, Kim was casted as Yoon Ji Hoo in the Korean drama Boys Over Flowers. Where he won Best Actor Award at the Seoul International Drama Awards 2009 and Popularity Award at the 45th PaekSang Arts Awards.
In 2009, SS501 released the album SS501 Collection, in which they each sang a solo song, along with being featured in a mini drama music video. Their Asia Concert Tour began in mid-August 2009 in Seoul at the Olympic Park. They continued their tour in Shanghai, Taipei, Bangkok before returning to Seoul for a closing encore concert. They staged two concerts for the encore closure on the 27th Feb and 28th Feb 2010 respectively.
In September 2009, during promotions in in Yokohama, Japan for Boys Over Flowers, Kim was diagnosed with H1N1. He was treated in a Toyko hospital and was discharged on 15 September 2009. While recovering from H1N1, he could not attend the Seoul International Drama Awards. To accept the award on his behalf, his members of SS501: Heo Young Saeng, Kim Kyu Jong, and Kim Hyung Jun were present for the award show.
He has also been the spokesperson for cosmetics brand, Tony Moly, an established company with numerous branches across Korea and Asia, including Taiwan and Hong Kong. He had since ended his contract with Tony Moly in End September 2010, and he had been chosen to be the Spokesperson for The Face Shop.
2010: SS501 comeback and leaving DSP Media
In April 2010, he appeared opposite Jung Ryeo Won in the music video for Korean Singer Gummy's new song "As A Man" (남자라서).
In June 2010, SS501 released an album titled Destination, produced by Steven Lee. Their title song "Love Ya" was well received, rating high in record sales and online music stores, and it went on to top Music Bank's K-Chart. However they ended promotions after two weeks, because SS501's contract with DSP Media ended.
On 19 June 2010, he then appeared at a concert in the Philippines along side BEAST.
On 28 June 2010, it was announced that Kim has officially left DSP Media and joined Bae Yong Joon's KeyEast Entertainment company. Although Kim has left DSP Media he stated that SS501's activities will be top priority in their contracts. and definitely has not disbanded. Kim explained in an interview that, at the time, there weren't a company that would accepted them as a whole, thus the group decided to take the chance to focus on their solo activities.
Kim played the lead role in the drama Playful Kiss, a Korean adaptation of Japanese shōjo manga Itazura na Kiss as Baek Seung Jo, who has a stand-offish personality. On the 7 November, Kim was interviewed by CNN's Anna Coren due to his success in Playful Kiss. Playful Kiss Special Edition is a big hit on Youtube as it is the first Korean drama to air on YouTube. Its first episode garners 1 million views just in 5 days and it proves so popular that there is a record channel view of 6.3 million and 17.5 million total upload views.
Kim was invited to perform at the 2010 Asian Games in Guangzhou as the representative of South Korea to sing "Sunshine Again" with other 3 other singers from Chinese-speaking countries. In the early of December 2010, Kim went to Singapore and Malaysia to promote The Face Shop.
2011: Solo artist
In May, Kim named his official solo fanclub 'Henecia', which is a combination of 'H' and the Latin term 'benecia', which means 'blessed'.
On 8 June 2011, Kim released his first solo debut mini album, Break Down. It was produced by Steven Lee, who composed SS501's hits, "Love Like This" and "Love Ya". The title track "Break Down", featuring Double K., is a high energy dance track, with a R&B heavy "Please", as the second lead track. The album exceeded 70,000 pre-ordered copies in 10 days. It peaked at number one on Gaon Weekly Album Chart for the week starting 5 June 2011 and was the best-selling album for June on Gaon Monthly Album Chart with 100,433 copies sold. It also top the Oricon chart in Japan in the International Imported Album category for the first week of July. The album's title track "Break Down" was the first place winner for two consecutive weeks on Mnet's M! Countdown music show, for 16 and 23 June 2011; and also won for two consecutive weeks on KBS's Music Bank. Break Down is certified platinum in Taiwan and Kim was presented with a platinum record by Warner Music Taiwan CEO Chen Ze Shan in a press conference held in Taiwan.
Kim was also the first Korean to appear on Taiwanese TV show The Person. In July 2011, he also held the first of his nationwide concert in Japan. In August 2011, Kim Hyun Joong starts his Asian Tour, and become the new Ambassador of Seoul.The tour continued from November in seven cities across Japan: Sapporo, Sendai, Hiroshima, Yokohama, Nagoya, Tokyo and Fukuoka.
On 11 October 2011, Kim released his second mini album Lucky. It debut at number 5 on Billboard's World Chart. It topped the Gaon Chart again for the third week of October, 2011 and was the highest selling album in Hanteo Album Sales Chart for the month of October, 2011. On 25 October 2011, Kim released a digital single together with Seohyun entitled "The Magic of Yellow Ribbon", which is the theme song of a commercial for The Face Shop. On 28 October, He along with Ha Ji Won acted as emcee at the 2011 K-Pop Super Concert held at Gwangalli Beach in Busan, which was broadcast on SBS on 6 November.
In November 2011, Kim won the 2011 KIA Soul People’s Choice award "Style Icon Award" for the third consecutive year. On 9 November Kim announced at a press conference for his Japan Tour that he has signed a deal with Universal Music Japan for his Japan-based music activities. Kim had successfully completed his Japan Tour that started on 9th of November, attracting over 30,000 fans. Kim won the Best Male Solo Artist in the 2011 Mnet Asian Music Awards that was held in Singapore.
On December 15th 2011, Kim released his digital album : Marry You/Marry Me. The digital album consists of 4 songs, namely Marry Me, Marry You, Marry Me (instrumental) and Marry You (instrumental). The music videos for Marry Me and Marry You have both been released on his official Youtube account on 15th December 2011 12 AM KST. On December 20, KHJ received 4 awards from Yahoo ( Yahoo Buzz Artist in Taiwan,Hongkong and Korea, and most search solo artist.