[5][6] Castellanos began his career working on the successful 1984 re-election campaigns for United States Senators Jesse Helms and Strom Thurmond.

During the campaign, Castellanos drew media attention for an ad he created that showed the hands of a white man holding a rejection letter because he had lost a job to a minority worker.

The ad, known as "Hands", was criticized by supporters of Helms' opponent, Harvey Gantt, for appealing to racial biases of white voters.

[26][27] Castellanos is currently a principal partner at Purple Strategies, major clients of the bipartisan firm have included the United States Chamber of Commerce, BP and PhRMA.

[4][24][28] Castellanos is a guest commentator and contributor for CNN[23][24] and has appeared on Meet the Press where he argued with Rachel Maddow in April 2012 about the existence of a gender gap in pay.

[5][31] In 2013, Castellanos shaved his mustache on The Situation Room with Wolf Blitzer after promising to do so if 500 new donors would give to Citizens United for Research in Epilepsy.

[32] Also in 2013, Castellanos led a Super PAC project called NewRepublican.org, which is focused on broadening the Republican Party's demographic base.

[33] Along with other Republican political consultants, Castellanos signed a "friend of the court" brief in support of gay marriage in February 2013.