The closest airport is Kisumu iternational airport
After Landing in Jomo Kenyatta international airport (NAIROBI), our visitors/volunteers/travellers, can use recommended transport, or any of these options:
1. Take  another plane to Kisumu international airport if you have booked prior, then either take a bus, Nisan or Probox from Kisumu bus stop to Mbita town through the shorter route of Lwanda Kotieno (Route/directions) or through the longer route of Homa-bay (Route/directions via Homa Bay)  
You can then take the  Mfangano water bus, Mfangano ferry or Mfangano Engine boats. all these first land at Sena Centre on Mfangano Island.
2. To those who are interested we do organize private Car through a transportion company in Kisumu, known as Amnitours to bring them direct from Kisumu airport to Mbita point which is so easy, faster and direct. You can then take the Mfangano water bus, Mfangano ferry or Mfangano Engine boats, all these first land at Sena Centre on Mfangano Island.
OR
3. Book any of these buses direct from Nairobi  to Mbita then to Mfangano through Water bus or Ferry (Route/directions) and choose either to travel night time or day time depending on their arrival time in Kenya. - (Easy coach, Ena coach, Transline, Classic or The Guardian) all stopping at their office doors in Mbita. This journey usually takes between 6-8 hours, and costs around 1500KShs  (11 USD). You can then take the Mfangano water bus, Mfangano ferry or Mfangano Engine boats. These land at Sena Centre on Mfangano Island. 
OR
4. Book another air plane from Wilson airport in Nairobi Kenya to Homa-bay air strip (Kabunde) then take either a bus, Nisan or Probox  from Homa-bay town to Mbita town route. You can then take the Mfangano water bus, Mfangano ferry or Mfangano Engine boats. These land at Sena Centre on Mfangano Island.