Debuting with the Adelaide Football Club in 2003 as a 19-year-old, Reilly took his time to develop, struggling to retain his spot in the side under the coaching of Gary Ayres. The appointment of Neil Craig as coach in mid-2004 significantly helped his development, with the youngster beginning to earn selection on a consistent basis. He played every home-and-away game in the 2005 season and the first final, before missing the last two with a hamstring injury, and ever since he has been a regular in the Crows' 22, renowned for his long and accurate kicking.
In 2009, with many younger players pressing their claims for selection, Reilly was one of a number of players with their places in the side under pressure, being dropped a number of times. However, he responded with a string of strong performances, and again entrenched himself in the Crows' best side. With the departure of Robert Shirley at the end of the season, Reilly began to make his name in 2010 in a new role as a tagger, most notably in the club's upset victory over Geelong, in which he kept superstar Gary Ablett to only 19 possessions.