The move by the Southern African Development Community (SADC), a regional bloc, was approved in September after Khoantle Motsomotso was shot dead at a barracks by officers from a perceived rival faction.
The shootout came just two months after elections and shattered hopes of ushering in a new era of stability in the volatile country.
Lesotho has been subject to several coups and periods of political unrest since gaining independence from Britain in 1966.
The seven-nation SADC force, which includes 207 military personnel, 15 intelligence officers and 24 police officers, will be deployed for six months and aims to bolster security.
“It is our firm conviction that the presence of this mission is necessary to establish a secure, stable and peaceful environment conducive for the implementation of … constitutional, parliamentary, judicial, public and security reforms,” Deputy Prime Minister Monyane Moleleki said at the official launch of the mission.
The SADC director responsible for politics, defence and security affairs, Jorge Cardoso, said the mission was in Lesotho to strengthen peace and security.
“(The SADC mission) is not here to take over or replace the Lesotho Defence Force or other security institutions,” said Cardoso, but to “support and complement” the government to attain lasting peace and security.
The force is drawn from Angola, Malawi, Namibia, South Africa, Tanzania, Zambia and Zimbabwe.