There are two airlines with regular service from Dar es Salam to Mafia's Kilindoni airport and flights take about 30 minutes.
Coastal Aviation
operates two daily flights from Dar es Salam and one "on-demand" from Kilwa, their largest plane is a 13-seated Cessna, flights cost USD110/person (http://en.wikipedia.org/wiki/Coastal_Aviation%7CWikipedia, (http://www.coastal.cc))Tropical Air
based in Zanzibar, flights to Mafia originate in Dar es Salam (http://www.tropicalair.co.tz)