Mozambique's President Samora Machel is to meet with President Reagan on Sept. 19, according to announcements here and in Maputo, culminating an unorthodox White House effort to strengthen relations with a Marxist revolutionary government in southern Africa.

Mozambican Ambassador Valeriano Ferrao said yesterday that Machel will arrive Sept. 17 on a week-long "official working visit" at Reagan's invitation and then will travel to New York to address the United Nations on Sept. 25.

The visit comes at a potential turning point in U.S. relations with Mozambique's neighbor, South Africa. A House bill that would place economic sanctions on South Africa's besieged white-minority government is expected to receive Senate approval next week, and the president is believed to be likely to veto it.

Machel, who signed a security accord last year with Pretoria, can be expected to speak out against the sanctions bill, according to a source involved in arranging the visit. Mozambique has upheld the security pact despite continued attacks by guerrillas South Africa had backed in the past. Machel has accepted South African assurances that the rebels now are relying instead on expatriate Portuguese seeking to recoup property lost when the colony became independent in 1975.

Administration efforts to entice Mozambique away from its close ties with the Soviet Union have included encouraging American private investment in Mozambique and initiating a small military-aid program. Congressional conservatives, opposed to dealings with a Marxist state, blocked the military aid -- which now is to be provided by Britain's Conservative government.

Machel's opposition to U.S. economic sanctions against South Africa is said to result from his experience with United Nations sanctions against neighboring Rhodesia's white-minority rule. Mozambique's troubled economy was linked closely to Rhodesia, now Zimbabwe, and was in financial stress as a result. Machel's economy is even more closely linked to South Africa.