Still relatively new to television, Miller won his first major role in 2007, when he played Kyle Brown in the long-running BBC children's show Grange Hill. He followed this in 2008 with his most prominent role, being the second actor to portray Aaron Livesy on Emmerdale. In late 2011 it was announced Miller was departing his role as Aaron, and his exit was broadcast in April 2012. He went on to have roles in Lightfields, Jamaica Inn and two series of Scott & Bailey before it was announced Danny was reprising his role as Aaron. Danny returned onscreen in August 2014.
Miller was previously in an on/off relationship with former co-star Kirsty-Leigh Porter between 2010 and 2015. In April 2016, it was reported that he had a secret fling with co-star Isabel Hodgins, which ended his relationship with Kirsty-Leigh, and put strain on Isabel's relationship with co-star Michael Parr.
In 2011, motivate by former producer Gavin Blyth's death, Danny and his friend Daniel Jillings set up a charity 'Once Upon a Smile' to support the families who have recently lost a loved one.