William Street is first recorded in Smyth's Directory, 1790 and was developed by 1791 according to Patton. However, the street existed much earlier and was recorded as Soap Lean on Maclanachan's map of 1715.
It is included in the list of streets in the Belfast Directory 1835-1836 with a reference to Church Street as an adjoining street.
Not to be confused with William Street South, which was never part of the same street.