Mariotti won in 1965, the Italian Youth Individual Championship. In 1969 he was single champion among adults and won the title of International Master. In 1971 he won again the individual national title and achieved the same time his first grandmaster norm. In 1974 he won at the Chess Olympiad in Nice a bronze medal as a single player on first board in the final total, as he participated in four Chess Olympiads. In 1974 he received the title of Grand Master. He was the first Italian to have managed since the introduction of title 1950.
For professional reasons, it declined as a consequence the number of its tournaments. From 1994 to 1996 he was President of the Italian Chess Federation. Later he worked as a supervisor of the national team.