Born and raised in Carlisle, Cumbria, the son of Debra Brewer and Leonard Miller, he became interested in athletics after working with Jack Harper-Tarr, a physical education teacher and athletics judge. Miller began training in throwing events at the local club, Border Harriers & Athletics Club, where Harper-Tarr coached. Miller preferred individual sports and also took part in karate, winning a national youth age-category competition. Initially, he competed in a variety of track and field throwing events, but he became increasingly interested in the hammer throw after seeing a rival youngster throw beyond sixty metres. Miller achieved this feat himself with the 5 kg implement in 2009, winning the Cumbria Schools Championships. In 2010 he won the English junior championships with a mark of with the 6 kg hammer. He repeated as champion the following year and also won the English Schools Championships title. After finishing at William Howard School, he gained an athletic scholarship to study a multi-disciplinary degree at the Oklahoma State University. In joining the American college, he followed in the footsteps of a fellow Border Harrier, distance runner Tom Farrell, who began studying there in 2010.
Move to Oklahoma
Miller began to compete athletically for the Oklahoma State Cowboys team and took up the weight throw event. His throwing coach, John Baumann, had trained several Olympians, including Gia Lewis-Smallwood. Miller was runner-up at his first major college event, the 2012 Big 12 Conference indoor championships. Throwing with the senior weight implement, he set a series of personal bests at the start of the outdoor season: he threw for second at the Texas Relays, before improving to, then. He was the champion of the Big 12 Outdoor Championship, becoming Oklahoma State's first conference hammer throw winner since 1901. In July that year he had another best with a mark of before going on to represent Great Britain in the qualifiers at the 2012 World Junior Championships in Athletics. He established himself as a top weight thrower in the 2013 indoor season, winning the Big 12 title before placing fourth at the NCAA Men's Indoor Track and Field Championship with a school record of. In the hammer he cleared seventy metres for the first time at the Mt. SAC Relays. He defended his Big 12 hammer title with a stadium record in Waco, Texas and won at the NCAA qualifier meet with a new best of. He gave his worst performance of the season at the NCAA Outdoor finals, finishing in ninth place with a sub-65-metre throw. He was similarly off his best at the 2013 European Athletics U23 Championships, where he also ended the competition in ninth place. He improved again in the weight throw the following indoor season, culminating a new best mark of, which brought him second place at the NCAA Indoor Championships and a British record. He also defended his Big 12 indoor title that season. He won a third straight Big 12 outdoor hammer title with a throw of – a personal best, school record, stadium record, and Big 12 Conference record. His winning margin was more than fifteen metres. He was runner-up in the NCAA Outdoor hammer throw that year, second only to Matthias Tayala.
Commonwealth medal
He won his first senior international selection for the 2014 European Team Championships and placed fifth for Great Britain. A week later, he was close to his best at the British Athletics Championships and won his first national title with a mark of. This led to his inclusion in the English team for the 2014 Commonwealth Games and at the event in Glasgow he threw to take the silver medal behind James Steacy – Miller's first international medal. In spite of this, Miller said "a little bit of me is a bit upset I didn't win but part of me is just 'what a fantastic achievement'".
