Why did the US support the tyrannical reign of Mobutu Sese Seko of Zaire?

Answer 1

For the same reason that the US often supported brutal dictators during the 1970's-1980's: because he opposed Communism and the Soviet Union.

Once the Soviet Union collapsed and the Cold War ended, the US pulled support for him and, despite some changes in his government, he was overthrown a few years later.

Answer 2

Not only did the US support Mobutu against Patrice Lumumba, the CIA actively tried to kill Lumumba by poisoning with in all probability president Eisenhower's full knowledge and authorization.

The reason was that Lumumba managed to antagonize his army within weeks of Congo's independence, resulting in nationwide revolts, riots and killings in which also whites (nuns among them!) were victims - which in the '60s immediately made him the devil incarnate in the eyes of white people everywhere. Mobutu. then Governor of the Katanga province, used the riots as an excuse to declare independence for his mineral-rich province . As he was well-connected to Belgian mining interests and seemed the 'reasonable alternative' to Lumumba (as well as the man who would protect and ensure access for the West to Katanga's minerals) he almost immediately got the USA's support.

Moreover, Lumumba made the mistake of calling for help and support from the Soviet Union against Katanga and his rebelling forces, thus threatening to turn - in the height of the Cold War at that - the Congo, Africa's biggest and most mineral-rich country, over to the Soviet sphere of influence. This confirmed the USA's worst suspicions about him and it sealed his fate. In a matter of weeks he had been abducted to Katanga and was shot by a firing squad there.