Goals by Mulota Kabangu and Dioko Kaluyituka stunned the South American champions as the team from DR Congo lived up to their nickname Tout Puissant (all powerful) to reach a final against Inter Milan of Italy or South Korea's Seongnam.
The African champions and their exuberant small band of fans who had played their brass music throughout the match joined in wild celebrations and dancing at the edge of the pitch.
"This win means a lot of things. All of Africa should be proud of these players. They have an enormous heart," the team's Senegalese coach Lamine N'Diaye said.
"After the opening 20 minutes Inter began to doubt. At half-time, with the score 0-0 our chances were intact. I said to the players the match was theirs for the taking, they had to go and find it," he told the post-match news conference.
The win saw Mazembe become the first African team to beat a South American side in the competition and a Brazilian side beaten for the first time in 13 matches at the Club World Cup in seven editions since it was started by FIFA in 2000.
Internacional coach Celso Roth said: "We had great expectations of this World Cup and greater responsibility having already won it once. It's a huge disappointment, we had chances, we dominated a lot but we lost.
"Our problem continues to be that we dominate but can't finish." Internacional won the trophy in Japan in 2006.
SHOCK LEAD
Kabangu put Mazembe ahead eight minutes into the second half when he was allowed space to take a headed pass and volley inside the far post past a static Renan.
Speedy winger Kaluyituka made the win safe for Mazembe five minutes from time when he turned a defender twice on the left before placing a low shot between the diving Renan and the near post.
Inter dominated the first half but were left to rue a string of missed chances, the best coming when keeper Muteba Kidiaba saved point blank from striker Rafael Sobis.
After the interval the impressive Kidiaba, who had led a prayer before kick-off with his team-mates kneeling on his goal-line, was on hand again to save again from Sobis.
Substitute Guiliano, who scored crucial goals in Internacional's Libertadores Cup campaign, looked to have made the breakthrough in the 70th minute only for his low shot to be turned away by the diving Kidiaba.
Tiring Argentine playmaker Andres D'Alessandro opted too often in the second half to play high balls into the box as Mazembe packed their defence and inched toward victory.
