Middlemass is originally a Scottish surname with a number of variants.
In his reference work "The Surnames of Scotland", George F Black includes the surnames Middlemas, Middlemass and Middlemiss as one and identifies the source of the name as an area around Kelso, Roxburghshire in the Eastern Borders of Scotland and specifically a place Middlemestlands. Further variations Midlemas and Middlemist appear in "A dictionary of English Surnames" by P H Reaney [subsequently revised by M A Wilson] where those authors also indicate the Scottish source of the surname.
The indexes of births, deaths and marriages registrations in England and Scotland will show further variations to include Middlemast, Middlemess, Middlemoss, Middlemost and Middlemus. There is a variation in the area of Northumberland, England around the chapelry of Beadnell in the parish of Bamburgh where the local parish registers show a name Middlemarsh [and at least once Middlemarch]. Black does not mention this surname in his publication. Reaney shows it as a separate surname altogether but both Middlemass and Middlemarsh appear in local registers within the same family and for the same person so this may be a result of local usage rather than an indication of the spread of that surname.
The 1881 census in the United Kingdom indicated that about 1,800 people had this surname in England, Scotland and Wales out of a population of about 29.8 million or 0.006%. As a percentage of the population of England, Wales and the Channel Islands it equates to 0.004%.
The 1911 census in England and Wales [including the Channel Islands, Isle of Man and servicemen around the World] is the first opportunity to examine the spelling of these variations of the surname in the handwriting of the families themselves, with one or two schedules completed by other household members. There are about 1,709 entries for Middlemass and these other variations divided as follows: Middlemas 270, Middlemass 283, Middlemast 49, Middlemis 8, Middlemiss 995, Middlemist 72 and Middlemost 32.
People with the surname Middlemass include: