The Algerian side led 2-1 from the first game in Dar es Salaam and, though they were beaten 1-0 by their unfancied opponents at home, lift the title on the away goals rule following the 2-2 aggregate draw.
Djuma Shabani (30) gave the visitors the lead from the penalty spot after seven minutes but they could not find a second goal that would have seen them crowned champions for the first time too.
Zineddine Belaid (24) had the chance to equalise on the night for USM but he missed his penalty and the hosts were left to cling on through the second half.
The closest USM had got to a continental title before was finishing as Champions League runners-up eight years ago, losing home and away to TP Mazembe of the Democratic Republic of Congo in the decider.
The final of this year’s Champions League will be contested between holders Wydad Casablanca from Morocco and 10-time winners Al Ahly of Egypt.
The first leg will be staged in Cairo on Sunday, with the second game set for June 11th in Casablanca.
