Mildren was signed by the
Florida Marlins as a non-drafted free agent on 19 February 2001. He split 2002 between the Rookie-level
Gulf Coast League Marlins and the
New York–Penn League Low-A
Jamestown Jammers. From 2003 through 2004 he was with the
South Atlantic League Single-A
Greensboro Bats. In 2005, he had a stellar year, pitching in the 2005
Baseball World Cup in the Netherlands for the
Australia national baseball team, and was awarded the
Florida State League Mid-Season All-Star on 18 June 2005, while pitching for the High-A
Jupiter Hammerheads. In 2006, Mildren continued his move up the Florida Marlins affiliate system to the Double-A
Carolina Mudcats, while also pitching for Australia again in the
2006 World Baseball Classic. With the Mudcats, he went 10–10 with a 4.14
ERA in 28
starts, led all Florida minor leagues in innings pitched, finished second in
strikeouts (150), and third in
wins, This led to Mildren being assigned to the Florida Marlins 40-man roster in 2007, and being promoted again to the
Pacific Coast League Triple-A
Albuquerque Isotopes at the beginning of the year. He split the season between the Isotopes and the
Omaha Royals when he was claimed by the
Kansas City Royals for their 40-man roster, off waivers from the Marlins on 17 August 2007. Mildren split his season three ways in 2008 pitching relief; first for the Omaha Royals, then the inaugural season of the
Texas League Double-A
Northwest Arkansas Naturals, and the High-A
Wilmington Blue Rocks, ending the 2008 season with a combined ERA of 6.62 over 32 games. and went to Prague as part of the
Australian team for the
2009 Baseball World Cup. ==Teaching==