After playing in the 2004 Claxton Shield he made his professional debut in the US minor leagues, with the GCL Phillies. He went an incredible 7–0 with a 1.75 ERA, walking a single batter in 612/3 IP while striking out 60. He led the Gulf Coast League in innings, tied Luis Valdez for the win lead, was three strikeouts behind leader Gabriel Sosa and was second to Gaby Hernandez in ERA. That earned him the nod as the All-Star starting pitcher for the 2004 GCL. Baseball America rated him as the league's No. 20 prospect, second on his club's staff behind Carlos Carrasco.[1]
Spending almost the entire season with the Batavia Muckdogs in 2005, he was 5–6 with a 5.35 ERA. In 2006, Scott was a more refined 1–1 with a 1.12 ERA for the Perth Heat in the 2006 Claxton Shield after having sat out the 2005 tournament.
Scott was 0–1, 2.77 for the Perth Heat in the 2008 Claxton Shield after having skipped 2007. He then was back with the Australian team for the 2008 Final Olympic Qualification Tournament, posting a 1.23 ERA in two relief stints, against Germany and South Africa with a save against Germany. Despite this, Australia failed to qualify.
In his first year in Oakland, Scott pitched for the Kane County Cougars (5–3, 1.74) and the Stockton Ports (1–0, 1.69). He made the Midwest League mid-season All-Star team. Mitchinson was on Australia's provisional roster for the 2009 World Baseball Classic and was a very late addition to their final roster after Ryan Rowland-Smith pulled out. He only threw two defensive outs in the Classic, but no runs were conceded off his pitching.[2]