Lumumba died Tuesday after being taken to St. Dominic Hospital with chest pains, the (Jackson) Clarion-Ledger reported.
"There were all kinds of complications going on, but ultimately, it was heart failure," Councilman Quinton Whitwell told the newspaper. "We're just all stricken with grief."
At the scheduled city council meeting held Tuesday evening, Council President Charles Tillman was so moved he could hardly speak, the newspaper said. He was elected interim mayor at the meeting.
Born Edwin Finley Taliaferro in Detroit, Lumumba was a political activist as a student at Kalamazoo State College and Wayne State University law school, where he graduated first in his class. He was involved in the defense of 16 Michigan inmates charged with murder after a prison riot and of Black Panther leader Geronimo Pratt.
Lumumba began practicing law in Mississippi in the early 1990s. He was elected to the Jackson City Council in 2009 and was elected mayor in 2013.