Peruvian election officials said that with 98.2 percent of the votes counted from the April 9 election, Humala led a field of 20 candidates with 30.7 percent, ensuring himself of a spot in a runoff likely to be held May 28.
He was followed by Garcia with 24.3 percent and Lourdes Flores with 23.7 percent.
Congressman Rafael Rey, a member of the National Unity alliance that supported the conservative Flores, admitted defeat.
"I think that we didn't manage to make up the vote difference that separated us from Alan Garcia, and for that reason Alan has advanced to the second round," Rey said in an interview with CPN radio Monday.
Rey said the alliance should not give "the impression it does not know how to lose," apparently referring to Flores' statement that it should wait until the final vote count before conceding.
Flores' inner circle "realizes that we have lost," Rey said.
Flores, 46, has had a hard time deflecting depictions of herself as the candidate of Peru's rich, white elite.
Garcia, 56, a dazzling orator, is seeking a second chance to govern after a 1985-90 presidency marked by hyperinflation and a bloody insurgency by Maoist Shining Path guerrillas.
Humala, 43, has frightened many middle- and upper-class Peruvians with pledges to radically redistribute the nation's wealth and increase government control over key sectors of the economy.