The Manse was always the home of a minister, and it was not necessarily next door to the church building.
In the 1850s, there were several different Presbyterian denominations, and all their ministers could possibly have been provided with a manse.
However, if a marriage took place in 'the Manse of Old Monkland' it is most likely to be the Church of Scotland minister's home.
One way to check would be to look up Old Monkland in 'Fasti Ecclesiae Scoticanae'. This will tell you the names of the ministers of every parish, and the dates of their incumbency. So if you find from the 'Fasti' that Rev John Johnston was the incumbent on the relevant date, that would answer your question. The 'Fasti' are available to consult online; Google is your friend.