Humans have known that lead is poisonous for at least 4,000 years. The earliest recorded account of lead poisoning was written by Nicander of Colophon in the second century BC.

History of lead poisoning
Ancient times: Lead was used in many ways, including in tools, utensils, and wine. Lead poisoning was common in workers of low social classes.
Roman Empire: Lead was used more extensively in the Mediterranean basin after the Romans conquered Britain. Some believe lead poisoning contributed to the fall of the Roman Empire due to it's use as a preservative in wine. Think of Caligula.
Industrialization: Lead poisoning became an epidemic during the industrial revolution. Big surprise to noone.
1970s: Concerns about low levels of lead exposure began.
1979: Herbert Needleman, a pediatrician and child psychiatrist, documented the dangers of even low levels of lead exposure. Symptoms of lead poisoning
Difficulty concentrating, Irritability, Learning disabilities, Behavioral disorders, Slowed development, Anemia, Seizure, Neurological problems, and Coma.

People have known that lead is poisonous in paint since the early 1900s. But, it wasn't until 1978 that lead-based paints were banned in the United States.

Early records
Hippocrates: Doctors have known that high doses of lead are poisonous since at least the time of Hippocrates.
Nicander: Nicander is credited with writing the earliest recorded account of lead poisoning in the second century B.C.E. Lead in paint
Early 1900s: Lead-containing paint was a main source of lead poisoning in children.
1940s: The lead industry began to reduce the amount of lead in interior paints.
1978: Lead-based paints were banned in the United States. Lead in other products
1878: Children were forbidden to work in white lead factories in the UK.
1883: The Factories (Prevention of Lead Poisoning) Act was approved in the UK.