The two are expected to sign the Egyptian-sponsored agreement Wednesday, marking an end to internal division and the establishment of a unity government, Xinhua reported Monday.
Abbas had arrived earlier in the Egyptian capital.
Mashaal and Abbas were to meet Monday and officials from both factions will attend meetings Tuesday and Wednesday before the signing of the agreement at the Arab League headquarters in Cairo, Bikyamasr.com reported.
Mashaal's deputy Musa Abu Marzouk warned that despite the agreement, Hamas will not recognize Israel.
"We won't recognize the Zionist entity because that would be at the expense of our stolen lands and our people in the refugee camps," The Jerusalem Post quoted him as saying,
Hamas Prime Minister in Gaza Ismail Haniyeh will also participate in the signing ceremony.
Israel Radio reported Monday that senior Hamas official in Gaza Mahmoud al-Zahar said the prime minister of the unity government should be a Gazan and not someone from the West Bank. This should be done for the sake of balance, he said.
"The reconciliation agreement between Fatah and Hamas needs the political and economic support of the Muslim Arab states so they can resist against the pressure exerted by the U.S. and the Zionist regime," Salah Bardawil a senior Hamas leader, told the Fars news agency.