Mario's role in the series was a rather large one. He was one of the main characters and appeared in almost every episode. He was probably the most important character.
Mario was also a huge factor in the Movie. He was probably the most important character, and even had an alternate form after he got the artifact (Fire Flower). He single handedly defeated Mr. L (Luigi's alter-ego) and had an alter-ego himself, Shadow Mario. He has appeared in every ACT, and every Part of every ACT.