1) Both a carpenter and an engineer, responsible for all the intricate wooden parts that were required in mills and mill machinery.
The OED has references to ‘mill wright’ from 1481-90, but this north-country equivalent has a longer history: 1386 Will. de Colburn, milnewright, York
1554 Joh. Flowere of Wheldrike ... husbandman and mylnewright
1585 William Hynchcliff of Holme, milnewright
1596 Millnwrightes and those that Cogg or Spindle any Wynd Myllne or Horse Myllne, Beverley
1678 Godfrey Hawkyard of Barkisland, milnewright.