Mito is connected to Tokyo via the Joban Line from Ueno Station. Limited express trains (Hitachi and Tokiwa services) make the journey in about 70 minutes; regular rapid services take around 80–90 minutes. This is the standard route and works very reliably.
There is no shinkansen service to Mito — the city sits outside the shinkansen network. Buses from Tokyo are an option but slower and not recommended when the train is this convenient. From Narita Airport, the Joban Line can be picked up at Abiko Station, though most travellers will find it easier to go into Tokyo first.
During the Mito Plum Festival (mid-February to late March), JR operates a temporary station called Kairakuen on the Joban Line for direct access to the garden. Outside of festival season, Mito Station itself is the terminus, and Kairakuen is a 20-minute walk or short bus ride.