Koh Mak is reached by boat from the mainland or from neighbouring islands. There is no airport — the nearest is at Trat, about 40 km away, with a few daily flights from Bangkok.
Speedboats leave from Krom Luang pier in Laem Ngop near Trat. The crossing takes 45–60 minutes and costs around 450 baht. Leelawadee and Boonsiri are the main operators; both have two or three daily departures in season.
Slow ferries are cheaper (around 200 baht) but take three hours and run only once or twice a day. They are a relaxed way to arrive if you are not in a hurry.
There are also direct speedboat connections from Koh Chang and Koh Kood, useful if you are island-hopping. Most resorts can arrange the transfer as part of your booking.