When Did Hasbro Acquire Monopoly? 1991 (via Parker Brothers)

Hasbro didn't buy Monopoly directly. They acquired Parker Brothers (Monopoly's publisher since 1935) in 1991.

• 1935: Parker Brothers publishes Monopoly (Darrow/Charles Todd origins).
• 1991: Hasbro buys Parker Brothers (and Tonka/Kenner) → owns Monopoly.
• 1999: Hasbro buys Wizards of the Coast (D&D, Magic).

So Monopoly became a Hasbro property in 1991 through the Parker Brothers acquisition.