Moving to Cardiff to be coached by Benke Blomqvist, Greene had his first success on the junior athletics circuit in 2005. He won the silver medal at the 2005 European Athletics Junior Championships, finishing with a personal best time of 51.14 seconds. The following year he competed in his first senior tournament, the 2006 European Athletics Championships, but he failed to progress beyond the heats with a run of 50.66 seconds. His age-group success continued, however, as he won the gold medal at the 2007 European Athletics U23 Championships with a new best of 49.58 seconds. He beat Frenchman Fadil Bellaabouss by a narrow margin to mark an impressive return, following an ankle injury which had ruled him out for six weeks of the season.