Millington made his first grade debut for Cronulla in Round 10 2008 against Brisbane. Cronulla went on to finish 3rd in 2008 but Millington made no appearances for the club during the finals series. In 2009, Millington made 17 appearances for Cronulla as they finished 2nd last on the table. In 2010, Millington played in nearly every game for Cronulla that year as the club finished near the bottom of the ladder.
Canterbury Bulldogs
In 2011, Millington joined Canterbury-Bankstown but only managed to make 7 appearances before being released by the club.
Ahead of the 2012 season, Millington joined English side Castleford on a two-year deal. He had an instant impact, making 22 appearances and scoring 4 tries in his first season. In July 2013, Millington signed a one-year contract extension with the Tigers, with head coachDaryl Powell labelling him "a stand-out performer". This was followed by a further contract extension in February 2014. Millington was voted as the Fan's Player of the Year for the 2014 season. Millington was named Directors' Player of the Year and 2nd Place Player of the Year for the 2015 season, of which he played every game. In 2016, due to the Tigers' extensive injury list, Millington displayed his positional versatility by starting 3 games at halfback for Castleford. Millington played a crucial role in the 2017 Castleford campaign, contributing to the Tigers' first League Leaders' Shield and Grand Final appearance. He played in the 2017 Super League Grand Final defeat to the Leeds Rhinos at Old Trafford. He was consequently named in the Super League Dream Team, and won the club's 2nd Place Player of the Year award. Millington was named 3rd Place Player of the Year for the 2018 season. At Castleford's 2019 end-of-season awards night, Millington picked up the Community Player of the Year title in recognition of his outstanding contribution to the local area and his hard work off the field in conjunction with the Castleford Tigers Community Foundation. Following his excellent start to the 2020 Super League season, Castleford head coach Powell stated that "this year he's at a different level" and expressed his intention to open further contract talks with Millington. Since signing with Castleford, Millington has turned into a fans favourite due to his consistency and commitment. He is utilised as a skilful, ball-handling and intelligent prop / second row forward and has even been called upon to cover in the halves on occasion. He is widely recognised by fans as one of the club's greatest imports.
