The Oarai-Kashima Line timetable is available here (Japanese only)
From Mito Station take the Oarai-Kashima Line to Oarai Station. The journey takes 15 minutes and costs ￥320. There are one or two trains every hour from around 6am to 11:30pm.
Take the Limited Express 'Tokiwa' or 'Hitachi' to Mito and change to the Oarai-Kashima Line. Limited Express trains cost ￥3,820 and take just over an hour to Mito.
Alternatively, take the Joban Line to Mito Station and change to the Oarai-Kashima Line. Joban Line trains take around 2 hours 30 minutes and cost ￥2,270.
(as of April 2019)
To/From Tokyo Station
The bus for Mito Station leaves from Tokyo Station Yaesu South Exit 2 - 3 times an hour and takes around two hours to reach Mito. Buses run from around 8am until midnight, however, late night buses must be reserved in advance. You can buy tickets online, on the bus or use the ticket machines in Tokyo Station Bus Terminal.
Buses to Tokyo leave from Mito Station South Exit.
A one-way ticket costs ￥2080. For more information and timetables please consult the Tokyo Station Expressway Bus Terminal Website here (available in 5 languages).
To/From Narita Airport
There is a direct bus from Narita Airport (all terminals) to Mito Station roughly once an hour. You can buy tickets at the Keisei ticket counter in Terminal 1 or 2. A one-way ticket costs ￥3100. Buses bound for Narita Airport leave from Mito Station South Exit.
For more information please consult the Ibaraki Bus website here (Japanese only)