Frustratingly little is known about the lost church of St Mary Mounthaw. It was one of the churches that was destroyed in the Great Fire of London and not rebuilt. There are no drawings of the church building, no church tower visible in the Hollar and Wyngaerde panoramas and no existing parish boundary markers (although the Museum of London is said to have one in their collection).
John Stow in his “Survey of London” in 1598 had only this to say:
” The parish church of St Mary Monte Alto, or Mounthaute; this is a very small church, and at the first built to be a chapel for the said house of the Mountehauts“
The Mountehauts were a wealthy Norfolk family who had large town house in London in medieval times.
We know where it stood. See the maps below from the Collectors Book and an old OS map. The first shows the area before Queen Victoria Street was built in 1861. The second is a later map from 1889. This part of London was completely obliterated by bombing in WW2 and has since been redeveloped, so there is nothing to show that there was once a church of St Mary Mounthaw there.
After the Great Fire, the parish was combined with that of nearby St Mary Somerset, of which only the tower survives today. If you go looking of the last physical remnant of the church you will find it far away in the outskirts of London, in the City of London Cemetery. There you will find the pictured memorial (below) to the churches of St Mary Somerset and St Mary Mounthaw. This is where buried human remains from those two churches were reinterred after their sites were redeveloped in the City.
Wikipedia page for St Mary Mounthaw here