Flying to Chiang Mai
Flying is how most visitors arrive. Chiang Mai International Airport (CNX) sits only a few kilometres from the Old City, so it's also the fastest way to get from "arrival" to "hotel room." Several airlines fly the Bangkok–Chiang Mai route multiple times a day, and there are also direct international flights from cities including Singapore, Kuala Lumpur, Hong Kong, Seoul and several Chinese hubs, with seasonal routes changing year to year.
A one-way domestic hop from Bangkok typically takes around 1 hour 15 minutes in the air. Budget carriers, full-service airlines and low-cost long-haul connections all serve the route, so prices vary a lot depending on how far ahead you book and the season — expect the widest range of fares and flight times around Thai public holidays and the Loy Krathong/Yi Peng festival period in November.
The overnight train from Bangkok
The State Railway of Thailand runs overnight sleeper trains between Bangkok's Krung Thep Aphiwat Central Terminal and Chiang Mai. The journey covers roughly 750 km and takes anywhere from 11 to 15 hours depending on the service and how many stops it makes, with second-class air-conditioned sleepers being the popular choice for the fold-down bunks and blanket service. It's slower than flying by a wide margin, but it's a genuinely scenic way to see rural Thailand, and it drops you close to the centre of town rather than at an airport.
Long-distance buses
VIP overnight coaches connect Bangkok's Mo Chit terminal with Chiang Mai's Arcade bus station in around 9–11 hours. They're usually the cheapest option and the seats on the better operators recline a long way, but road journeys of this length are tiring compared to a short flight, and journey times stretch out further around holiday traffic.
Coming from elsewhere in Northern Thailand
If you're already travelling around the north, minivans and buses link Chiang Mai with Chiang Rai, Pai and Mae Hong Son, and these routes are a normal part of a longer Northern Thailand loop rather than a one-off transfer.
If your only priority is minimising travel time, fly. If you want the overnight train or a scenic bus/minivan leg to be part of the trip itself, budget an extra day either side so a delay doesn't eat into your Chiang Mai time.