Mari is universally considered to be one of the greatest theorists of Italian and world design. Since the 1950s, he has participated in avant-garde design movements, joining the group of Kinetic Art: there, he met Bruno Munari, who was to influence some of his future works, namely 16 animals and 16 fish.
 
In 1963, he became coordinator of the Nuova Tendenza Group and organised the group’s display in the Zagreb Biennial of 1965. From 1963 to 1966 he taught at the Scuola Umanitaria in Milan: this was just the first of his teaching experiences that continued through to 2000 or so in a great many prestigious schools including the Politecnico di Milano University, where he ran courses in the Industrial Design and Architecture faculty, and Parma University, where he taught History of Art. During these years, he elaborated his own theory of design, putting it into practise in the projects on which he worked in product, graphics and fit-out sectors. He was rewarded for this continual study work in 1967 with a Compasso d’Oro for his ‘individual research into design’. Again in 1967, he began his ten-year collaboration with Gabbianelli: against a request to design some decorations for ‘design tiles’, Mari refused to play a simple role as graphic and undertook a complex philosophical path that led him to review the very roots of wall decorative concept. For his tiles (1968 Elementare series, 1978 Traccia and 1981 Decorato a Mano to mention just some of the most important), Mari recovered technologies of the past, developing a poetry of shapes and colours made up of basic symbols. In 1974, Mari published Funzione della ricerca estetica, where attention moves from the debate on the product design to the figure of the designer. In 1972, Mari participated in the Italy – The New Domestic Landscape exhibition at the New York MOMA: the exhibition, which was extremely important and marked the birth of the famous ‘Made in Italy’ worldwide, contained items by the greatest designers of the time, such as Vico Magistretti, Ettore Sottsass and Paolo Lomazzi. Mari displayed his reversible vase, Pago-Pago (1969) by Danese at the exhibition - a moulded ABS vase that could be used straight or upturned, changing the aesthetic effect: the basic idea was to grant flexibility of use, given the impossibility of creating the perfect design for every ambiance. From 1976 to 1979, he was chairman of the ADI, the Industrial Design Association famous for assigning the Compasso d’Oro design awards. In 1983, Parma University devoted a personal exhibition to him, thanks to a collection of 8,500 sketches and original drawings donated by Mari to the University’s Communication Study and Archive Centre. Mari’s works are displayed in the world's most important art and design museums, including the Galleria Nazionale d’Arte Moderna in Rome, the New York Museum of Modern Art and the Paris Louvre. In 2001, he published Progetto e passione. In 2008, the Galleria di Arte Moderna e Contemporanea in Turin devoted a personal anthology to him on the occasion of the International Year of Design.