Mechai Viravaidya is the founder and Chairman of the Population and Community Development Association (PDA), one of Thailand's largest private, non-profit development organizations. Since 1974, PDA has initiated community-based fami planning services, community and rural development programs, and HIV/AIDS prevention activities throughout Thailand. The success of Thailand's national family planning program resulted in one of the most rapid fertility declines in the modern era as the rate of annual population growth declined from 3.2% in 1974 to 1% in present, and the average number of children per family fell from seven to two. 

He served as a Senator, a Minister of the Office of the Prime Minist er twice, in 1991 and 1992, when he assisted the Former Prime Minister Anand Panyarachun in establishing a comprehensive national HIV/AIDS prevention policy and program. He has also served as Government Spokesman, Deputy Minister of Industry, Governor of the Provincial Waterworks Authority, Chairman of Krung Thai Bank Public Company Limited and Telephone Organization of Thailand. He was appointed as the Ambassador for UNAIDS in 1999.