Mendoza joined the Concerned Citizens Against Pollution (COCAP) in 1995. At that time, COCAP was campaigning for clean air, as there was a plan to put up an incinerator plant in the country. This was her first involvement in environmental issues. She attended hearings with a group of ladies led by Odette Alcantara at the Senate and at the House of Representatives when incineration issues were taken up. She also joined rallies at the Luneta, campaigning for clean air and for the passage of the Philippine Clean Air Act that bans incineration.

In 1998, Mendoza and four other housewives decided to take action when the garbage collectors failed to collect garbage in their village for three weeks. With Luz Sabas and Dr. Metodio Palaypay serving as their gurus on proper waste management, they started a waste segregation campaign. “The biodegradable [nabubulok] waste was composted and the non-biodegradable [di nabubulok] ones were sold to the junk shops to be sent back to the factories for recycling,” Mendoza said.

In 1998, a group of five men and 10 women environmentalists founded the MEF, and registered it at the Securities and Exchange Commission. Since then, the MEF has been teaching communities, schools, offices, and various sectors of society how to do recycling and proper waste management.