Leeflang started playing the drums at age 13, playing along with records of bands such as Nirvana, Guns N' Roses, Metallica and Iron Maiden. He started playing with local hard rock bands. In 1996, he joined Dutch gothic rock band Within Temptation at age 17 as their first official drummer.[1]
Leeflang played with various other Dutch rock and metal bands between 1997 and 2004, with one notable band being the Saturnine, before relocating from the Netherlands to New York City, shortly after having departed his band Sun Caged, of which he was a founding member. In 2006, Leeflang rejoined the Saturnine as an inter-continental recording project with occasional live performances in Europe.
Leeflang had been Ron "Bumblefoot" Thal's live drummer for European tours since 2002, but became his full-time drummer after moving to New York in 2004. Leeflang recorded a song with Bumblefoot in 2003, but his first full album with Bumblefoot was Normal in 2005. This was followed by the 2008 album Abnormal. In 2015, Leeflang and Bumblefoot released the album Little Brother Is Watching. With Bumblefoot, Leeflang toured extensively between 2002 and 2015.
When not touring, Dennis works out of his recording studio in Los Angeles, California, as a session drummer and recording engineer.
Dennis is also a founding member (along with Chris Szkup and Richard Anderson), Drummer and Mixing Engineer for the worldwide studio project The Feckers,[3] some of the people who have recorded with The Feckers include Ron "Bumblefoot" Thal, Neil Murray, Tommy Denander, Richard Fortus, Brett Garsed, Kyle Whalum, Melvin Brannon Jr, Lydia Salnikova, Tony Franklin, Joey Huffman, Jonathon Noyce, Alessandro Bertoni, Chris Sligh, Tūranga Morgan-Edwards, Kyle Thomas, Zak Lloyd.
