In 1983, Carlos founded Sin Fronteras Organizing Project to support efforts to improve the working and living conditions of the migrant and seasonal farm workers of Southern New Mexico and Far West Texas. Marentes is also the founder and director of the Border Agricultural Workers Project, an effort to organize the farm workers of the US-Mexico border. He participates in many local, state and national organizations that deal with issues of poverty and economic inequality, and coordinates the International Committee on Migration and Rural Workers of La Vía Campesina and has attended many conferences and workshops in U.S. as well as in Mexico, Europe, India, South Africa and Southeast Asia, to advocate for migrant workers rights.