Grimace first appeared in McDonald's advertising in 1971.
The character originally had a more mischievous identity and was known as the Evil Grimace. Early advertising portrayed him with four arms and a role connected to stealing milkshakes, unlike the friendly purple character most audiences know today.
McDonald's later redesigned Grimace with a simpler, softer appearance and made him one of Ronald McDonald's recurring friends. That redesign helped establish him as a recognizable part of the McDonaldland cast.
People sometimes date Grimace's arrival to his later friendly personality, but the character's first advertising appearance was in 1971.