Hansen is the founder, lead guitarist, and vocalist of power metal band Gamma Ray. He is also the co-founder and former member of German metal band Helloween. He is a prominent figure in power metal and has sold millions of albums worldwide. He is regarded as "the godfather of power metal", having founded three seminal bands in the genre. In 2011, he joined the band Unisonic featuring former Helloween vocalist Michael Kiske.
Hansen played the role of Regrin the dwarf in Avantasia's Metal Opera albums by Tobias Sammet. Later he was featured in the album The Scarecrow playing guitar in the song "Shelter from the Rain". He was also one of the guest musicians in Avantasia's 2010 world tour, which have put him to play live together with fellow ex-bandmate Michael Kiske, after more than twenty years since he left Helloween. After this tour it was announced, via Michael Kiske's official website, that Hansen would join his new band Unisonic, as they "instantly felt the magic of Helloween years again and had a lot of fun together on and off stage".
In 2005, he toured with the young German power/Viking metal band Stormwarrior from his home town of Hamburg as lead singer, playing material from his classic Helloween debut album "Walls of Jericho" – the favorite album and greatest influence of Stormwarrior. "Stormwarrior featuring Kai Hansen" returned in 2007 to play at the Magic Circle Festival in Bad Arolsen and at the Wacken Open Air Festival.
In 2011, Kai joined the hard-rock band Unisonic, as second guitarist and reunited with old friend and ex-Helloween vocalist Michael Kiske, who formed the band in 2009. The band released its debut album, Unisonic in 2012 and embarked on its first world tour. Hansen stated in an interview that despite being a member of Unisonic, he will not have less focus on Gamma Ray, and that that the next Gamma Ray album should arrive in 2013. Unisonic's second album, Light Of Dawn, was released in 2014 and a European and Japanese tour followed.
During his earlier years Hansen's singing style was reminiscent of many thrash metal vocalists. Rather than using a smooth and clear style (like his successor Kiske) Hansen sang with a raspy sound, interluded by high-pitched screams for dramatic effect. Kiske was brought in as a singer originally because Hansen had trouble singing and playing the guitar simultaneously though later in his career he seems to have overcome these problems.
Beginning from 1995 onward, Hansen has provided the vocals for his current band Gamma Ray. Around that time, Hansen's vocal style became more melodic and slightly clearer, more in the traditional power metal sense, while still retaining the slight nasal tone and rasp of his early years.
For almost 20 years, Hansen has played ESP Guitars. He first started using them during his days with Helloween after hearing testimonials of ESP from artists such as George Lynch and Kirk Hammett of Metallica. Before Helloween, he had owned a white Ibanez guitar, and a Les Paul, then saved up for a Marshall amplifier by delivering newspapers. His main ESPs are red and pink ESP Custom guitars based on Jackson's Randy Rhoads model and two custom V models based on the classic Gibson Flying V shape. During the early 1990s, Hansen had used Stratocaster models. On his autobiography on the Gamma Ray website, he claims his first electric guitar was a white Ibanez Les Paul copy. On the recent tour with Gamma Ray he has been seen using a Korina Epiphone flying V with EMG's.