There are several flights daily from Land's End Airport, near Zennor, and from Newquay Airport. Adult fares start at £80 one way. Summer flights also run from Exeter, Bristol and Southampton. Since 2018 there has also been a helicopter service from Land's End Airport to St Mary's. The flight time is around 20 minutes.
There are also special day-trip fares (£119), or you can also choose a fly-sail combo ticket (£88).
There is parking at Land's End airport, but if you're staying on the islands a while, it's much cheaper to book a minibus transfer from Penzance railway station for £6 each way.