In the mid-2000s, Mateo emerged as a leading Republican activist and fundraiser. In 2002, he raised $400,000 for New York Governor George Pataki's re-election campaign. In 2003, he was a major fundraiser for President George W. Bush, who named him to the White House Commission on Presidential Scholars. In 2004, he spoke at the Republican National Convention. In 2005, Mateo served as Hispanic Director for the re-election campaign of then-New York City Mayor Michael Bloomberg.