He studied violin from 1929 until 1935 at the Conservatory of Liepāja, then composition and conducting (under Leo Blech) at the Conservatory of Riga from 1940 until 1944 while working as violinist at Riga Opera.
In 1944 he was appointed conductor of Riga Opera, then of the Latvian Radio Orchestra (1947–1952).
Jansons became principal guest conductor of the Hallé Orchestra in 1965.
He collapsed and died from a heart attack in 1984 while conducting a concert with the Hallé in Manchester.
He is buried next to Karl Eliasberg in Volkovo Cemetery, Saint Petersburg.