He was killed in the 1999 Armenian parliament shooting while serving as Deputy Prime Minister of Armenia.

Leonard Petrosyan was born in Martuni, Nagorno-Karabakh, Azerbaijan SSR, where he attended the local primary as well as secondary school from 1958 to 1970.

After graduating as an economist in 1979, he continued working in the system of common nutrition in an agricultural center run by the Martuni regional community of the Communist Party, a position he held since 1975 and kept until 1988.

Petrosyan became the President of the National Assembly of Nagorno-Karabakh, a regional council, on 12 September 1991.

He left the position on 8 January 1992 and became the first deputy head of the department of special programs of Armenia.