Thank you for your question and welcome to Just Answer.
At the Federal level, marijuana became illegal on August 2, 1937. Here is a rather lengthy site about marijuana becoming illegal. The source is slanted towards legalization, but it is packed with dates and information.
Another source here.
I hope this has helped! If so, please don't forget to "Accept!"
Regards,
Ryan
Professional Researcher
I can find answers to even the most obscure legal question! Satisfaction guaranteed!