Ronald McDonald was the McDonald's mascot introduced in 1963 before Grimace and the Hamburglar became famous.
Ronald first appeared in a television commercial in 1963 and soon became the central human character in McDonald's advertising. His clown appearance connected the restaurant brand with children and helped create a more elaborate world around the company's restaurants.
Grimace and the Hamburglar were introduced later as part of the McDonaldland character group. Other characters, including Mayor McCheese and Birdie, also appeared in subsequent advertising. Ronald remained the principal mascot and the public face most strongly associated with the brand.
This chronology is sometimes blurred because the characters were later shown together in campaigns. Ronald's 1963 debut, however, predates the introductions of the other major McDonaldland figures named in the question.