DDT was first synthesized in the 19th century and used extensively in the 20th century, when it's effectiveness as a pesticide was discovered. Below are links to sources detailing its history.

DDT - From Wikipedia, other versions: français, español, russe

A Century of DDT (extract)

Oregon State University, USA - History of Pesticides Use

US Environmental Protection Agency - DDT Regulatory History: A Brief Survey (to 1975)