Stephens joined Radio 1 in 1999 at the age of 17 as part of the station's new regional output, where he hosted the Wales opt-out with Bethan Elfyn and became the youngest ever Radio 1 presenter. Before this he was a DJ on Rookwood Sound Hospital Radio in Llandaff, Cardiff.
In 2005, Stephens gained a national slot when he became one of the replacements for the late John Peel as part of Radio 1's One Music strand, which was intended to keep the spirit of Peel's show going with DJs Rob da Bank and Ras Kwame. He went on to present the weekend afternoon show and a two-hour new music programme on Thursday evenings, as well as being regular daytime cover. Stephens now presents the 22:00–01:00 show, Monday to Wednesday.
Stephens is a fluent Welsh speaker and as well as on Radio 1, he is also a presenter of BBC Radio Cymru's music strand C2 on Monday evenings, previously at 10 pm and currently at 7 pm (although the slot is broadcast during summer 2015 by Gwyn Eiddior while Stephens broadcasts on Saturday afternoons). Stephens also presented Bandit, a television show on Welsh-language channel S4C which showcased Welsh language music talent, until it was taken off air in December 2011. He founded the "Boobytrap Singles Club", which released limited singles by Mclusky, The Keys and Zabrinski amongst others. Stephens is one of the founders of Welsh language label Am, whose releases include Mr Huw, Radio Luxembourg and Threatmantics. In 2009 he released a compilation called "Music Sounds Better With Huw Volume 1" via Wichita Recordings on iTunes. It featured 21 up and coming artists including Gold Panda, Dinosaur Pile-Up, Banjo or Freakout and Young Fathers.
Stephens also presents a weekly Radio 1 free podcast. He curates the Introducing Stage, which showcases new unsigned and emerging music at the Reading and Leeds, Glastonbury, Radio Ones Big Weekend and T in the Park festivals, also invited bands and artists to play at Sonar and Groningen. Since its first year he has curated the Lake Stage at the Latitude festival in Suffolk. Stephens is the opening presenter on the Mountain Stage at the Green Man Festival and introduces and start of the festival and music program.
In 2005 Stephens became the compere and DJ for the main stage of Reading Festival. He has DJed at Green Man, Sonar, Camp Bestival, Big Chill, Field Day and the National Eisteddfod of Wales. He has contributed to the Western Mail, Kruger Magazine, The Independent, The Mirror and NME, and guest-edited the Guardian music blog. He is the voice of the Discovery Shed television channel.
In 2007 he established a new music festival for Cardiff. The first Sŵn festival took place on 9–11 November across 13 venues in the city and has occurred yearly since. Swn (pronounced "soon") is the Welsh word for "sound", and the festival brings some of the most exciting new bands, DJs and performers from Wales, the UK and elsewhere together with art and films in Cardiff city centre's venues. The organisation also arranges gigs in Cardiff throughout the year.
In 2015 Stephens began co-presenting the RTÉ music show Other Voices, featuring performances from the Irish festival, alongside regular presenter Aidan Gillen. He was also the host of that year's BAFTA Cymru awards ceremony, which took place at St. David's Hall in Cardiff in September.
Stephens was born in Cardiff, Wales, the son of the author and literary journalist Meic Stephens. He married his longtime girlfriend, Sara, in 2012.
In July 2015, Stephens received an Honorary Fellowship from Bangor University. Among those also receiving the honour that day was his cousin, musician Gruff Rhys. Their mutual grandfather was president of the students union at the University.