The grenade "hit the tail end" of the chopper, said Major Martin O'Donnell, a spokesman for Nato's International Security Assistance Force.
"There was a minor damage to shaft and the rear blade," O'Donnell said.
"The helicopter landed under control" at a nearby Nato base.
Mangal and other officials were to inaugurate a new mosque in Musa Qala, a town that lies in Helmand at the heart of the country's opium poppy-producing region.
US, British and Afghan troops pushed Taliban fighters out of Musa Qala late last year after the fighters overran the area in early 2007 and held it for 10 months.
The delegation returned to the provincial capital of Lashkar Gah in another helicopter, Mangal said.
Because of Afghanistan's bad terrain and lack of proper road infrastructure, helicopters are essential for transporting people and equipment for foreign and Afghan troops battling a Taliban-led rebellion.
The Taliban government was ousted in the US-led invasion in 2001 for sheltering al-Qaeda leaders.
Separately, a roadside blast hit a vehicle in eastern Paktia province on Saturday, killing three civilians, Ghamai Mohammadi, a government spokesman, said.
A bomb placed on a bicycle exploded as a police vehicle passed by in Kandahar city in the south, killing a 10-year-old boy and injuring another civilian, Mohammad Nabi, a police officer, said.
No police casualties were reported.
Fighters regularly use roadside bombs against Afghan and foreign troops in the country, but most of those killed in such attacks have been civilians.
More than 1,200 people - mostly Taliban fighters - have died in insurgency-related violence so far this year, according to a tally compiled by the Associated Press.