Matti Lehtinen studied solo singing at the Sibelius Academy in 1945-1948, after which he went in 1949 to the opera class at the Stockholm Royal College of Music. He also had private studies with Paul Hansen and Elbe Nissinen.
Matti Lehtinen made his concert debut in Helsinki in October 1948, and his opera debut at the Finnish National Opera in 1949. Subsequently, he received his first engaement at the Finnish Opera in 1949-1951. In 1950 he won 1st Prize at the Geneva International Singing Competition. He was engaged at the Cologne Opera House from 1952 to 1955. Thereafter, during in the years 1955-1963, he preferred to work as a free-lance concert and oratorio singer, which also consisted of Lieder singing.
In the autumn of 1963, Matti Lehtinen, was again engaged at the Finnish National Opera. The same year he was appointed as Professor of singing at the Sibelius Academy, a post which he held until 1987. He was also Director of the Academy opera class from 1963 to 1969. he continued to appear as a regular visitor at the National Opera, after he has a professorship at the Sibelius Academy, which allowed him to keep the two posts. He was greatly acclaimed both in his homeland and abroad. In 1965 he was awarded the Pro Finlandia Medal.
At the National Opera and the Savonlinna Opera Festival, Matti Lehtinen performed many memorable baritone roles, including Papageno in The Magic Flute, Don Alfonso in Cosi fan tutte, Leporello in Don Giovanni, the title role of Juhan in the opera by Aarre Merikann, Escamillo in Carmen, Porgy in Porgy and Bess, and Johannes Ponttoon in Kalevi Ahonís monologue opera Avain (The Key), which he performed in 1979. He also performed Erik Bergman's choral-orchestral works
As a singing professor, Matti Lehtinen trained a number of promising singers, such as the bass Jaakko Hietikon and the baritones Jaakko Kortekankaan and Esa Ruuttusen. Similarly, Elja Puukko and Heikki Raivio were also his students. As Director of the Opera Studio he also had many students, who would become a successful singers, such as the famous bass Matti Salminen.