He became known in the German hip hop scene through his collaboration on Carlo, Cokxxx, Nutten and Bushido's album Vom Bordstein bis zur Skyline. Losensky's debut album Neue Deutsche Welle caused controversy with the media, who criticted its lyrics as right-wing extremist.
Losensky has engaged in feuds with other rappers including Eko Fresh, Bushido and Kollegah.
Losenski grew up in West Berlin without knowing his father. He has stated that he had problems in school, not because of his grades, but because of his behavior. At the age of 15, he went to a psychiatric hospital because of a panic attack. He was later institutionalized. During his stay, he began an apprenticeship as a painter and decorator, were he met Bushido and both became friends, but later he broke off this pursuit. Fler made a name for himself in the Berlin graffiti scene. Fler, who describes himself as hyperactive, saw a career as a rapper as an escape from crime and financial difficulties.
2001-2004: Early career and Carlo, Cokxxx, Nutten
In October 2002, he became known through his collaboration with Bushido on Carlo, Cokxxx, Nutten (under the pseudonyms "Sonny Black" and "Frank White"). The contest of most of the song from the album revolves around the life in the ghetto, where both rappers thermes drug dealing, prostitution and gun violence. Because of the gloomy production and samples counts Carlo, Cokxxx, Nutten to one of the influential German hip hop albums.
In 2003, Fler had been featured on six songs on Bushido's album Vom Bordstein bis zur Skyline and signed to Aggro Berlin as fourth artists in October. His debut single, "Aggroberlina" (released 2004) hit #59 on the German charts.
In 2004, Bushido left Aggro Berlin after a dispute and formed his own label, ersguterjunge. He offered Fler to join him, who refused. The two have since broken contact.
2004-2005: Neue Deutsche Welle
Fler released his first solo album Neue Deutsche Welle, on 1 May 2005. The first single was "NDW 2005", with a sample of Falco's hit single "Rock Me Amadeus". Due to supposed right-wing extremist lyrics, the album was very controversial.
2006-2007: Trendsetter and Airmax Muzik
In 2006, he released a second solo album entitled Trendsetter. The first single "Papa ist zurück" reached in Germany number 23 and in Austria number 32. The second single "Cüs Junge" wasn't very sucesessful, it reached in Germany number 50 and in Austria number 75.
On 2 February 2007, Fler released his mixtape Airmax Muzik, which included disstracks and two music videos for the songs "Was is Beef?" and "Das is los!". In April 2007 some dates on the Airmax Muzik-Tour were canceled due to illness.
2008-2009: Fremd im eigenen Land, Südberlin Maskulin, Fler and Carlo, Cokxxx, Nutten 2
On 25 January 2008, Fler released his third solo album Fremd im eignen Land. He explained in an interview that after the release of the controversial and the Nazi reproachfulness in Neue Deutsche Welle, this time around he wanted to emphasize technique and flow instead of provocative lyrics. The first single "Deutscha Bad Boy", released the 11 January and reached place 16 in the single-charts. In the second week the song placed the 31 place of the hitparade. In the first week the album reached place 7. It stayed six weeks in the top-100. The second single "Warum bist du so?" (released 29 February), reached place 61 and stayed three weeks in the top-100 of the single charts.
Later in 2008 he released the album Südberlin Maskulin (a reference to Westberlin Maskulin) together with Godsilla. For Godsilla this was the most successful release of his career. In Germany the album reached place 22 and in Switzerland #81 and sold over 15000 copies. The only single was "Ich bin ein Rapper".
On March 27, 2009, he released his fourth solo album Fler. The singles were: "Check mich aus" and "Ich sing nicht mehr für dich". After the release, he left Aggro Berlin and the label closed down on 1 April.
Bushido contaced Fler, shortly after the closing of Aggro Berlin, wanting to end their feud. They officially showed their reconciliation with a photo shooting and an interview for Bravo HipHop Special. The only single "Eine Chance/Zu Gangsta", released the 28th August 2009 and reached #26 in Germany and Austria. The album Carlo, Cokxxx, Nutten 2 released on September 11. It reached succesessful #3 in the Media Control Charts. "Honestly I've never believed it. With all that competetion... and totaly without digital releaes...without big promotion", wrote Bushido on the album's website.
Fler's fifth album titled Flersguterjunge released on June 2010 over ersguterjunge. Lead single "Das alles ist Deutschland" (feat. Bushido and Sebastian Krumbiegel from Die Prinzen), released on June 4. The music video for the song had released the day before as premier on the internet. "Schwer erziehbar 2010" released as second digital single.
Together with Bushido and Kay One, re-formed Fler rap group Berlins Most Wanted, that had been founded in 2001 by King Orgasmus, Bass Sultan Hengzt and Bushido. On 22 October 2010, released the groups album titled Berlins Most Wanted. It reached in the German charts #2. The only single also titled "Berlins Most Wanted" reached in Germany #31 and in Austria #57.