Roberto Madrazo, who served as head of the Institutional Revolutionary Party, or PRI, received just over 92 percent of the vote, while the only other candidate, little-known Everardo Moreno, captured about 8 percent, with 28 percent of voting centers reporting, said Mariano Palacios, the party's president.
Official results won't be available until later in the week, and while it was unclear how many party supporters voted, officials acknowledged that the race was marred by low turnout.
Hundreds of Madrazo supporters packed the party's headquarters in Mexico City and began celebrating even before preliminary results were released.
The primary marked the first time the PRI has chosen a presidential candidate without controlling the presidency, and though the party has been marked by division, Moreno was never expected to mount a serious challenge.
Madrazo, an old-style PRI power broker, voted Sunday in his native Tabasco state, where he was governor. He later denied charges that the primary was not a serious democratic exercise and said that reports of division within the PRI were exaggerated.
"I'm sure that after today, we can win the internal election and things will get better for the life of the party," he said.
The PRI controlled Mexico's presidency from its founding in 1929 until President Vicente Fox was elected in 2000. Fox is barred from running again and disappointment with his conservative National Action Party has become widespread after the president failed to make good on many campaign promises.
Madrazo's bid for the PRI nomination was not derailed by the opposition of a powerful, 1.2 million-member teachers union which had threatened to boycott the primary.
The union usually counts itself among the PRI's strongest supporters, but a political spat that pitted Madrazo against union leader Elba Esther Gordillo led many teachers to oppose Madrazo's candidacy.
Gordillo quit the party's No. 2 post in September after courts rejected her argument that PRI bylaws should have let her follow Madrazo as party leader. Madrazo stepped down in August to run for president and was replaced by Palacios.
Besides National Action, the PRI bid to regain the presidency will have to contend with former Mexico City Mayor Andres Manuel Lopez Obrador of the leftist Democratic Revolution Party, who leads most polls ahead of the election in July.

