Thomas made his AFL debut in Round 1 of the 2007 season, scoring no goals but five behinds in a loss against Collingwood.
In 2010 Thomas led North Melbourne's goalkicking with 29 goals which included a bag of 7 against Carlton in round 12.
2011 wasn't s successful season for Thomas, as he struggled badly with bad kicking for goal all year and was at one stage dropped from the senior side because of it. He finished the season with 21 goals and 36 behinds.
In 2012, Thomas' accuracy improved markedly, as he kicked 38.19 for the season. The total of 38 goals for the season was his highest in a year so far. He played 19 games for the year, and was held goalless in only 3 of them. He played his 100th game in Round 12, 2012, against Gold Coast. In September 2012 he signed a two year deal with the club to remain there until the end of 2014.
In 2013, Thomas had a career-best year, kicking 53 goals for the season. He also finished seventh in the Coleman medal, the highest position for a small forward that year.