Mennim married Dr Eleanor Wilson in 1952 and joined the practice of T P Bennett and Partners in London, where they had many large-scale commissions.
He took part in jungle warfare in Burma during the advance on Mandalay and was involved in planning for the invasion of Malaya when peace was declared in August 1945.
Ferrey and Mennim were Diocesan Surveyors for the immense Yorkshire diocese which meant that they had under their care around 300 churches of varying ages, each of which had to be fully inspected every five years.
In the late 1960s Mennim went for an interview at University College, Cambridge where his friend Hugh Plommer was a Founding Fellow.
This was the start of a close working relationship between Mennim and the College of which its greatest fruit was the collection of buildings of pleasing symmetry, constructed in hand-made Stamford bricks with the handsome Main Building in Weldon Stone, all arranged in well laid out courts on the Bredon House site between the years 1972–77.
There had been many false starts in the early years when Michael had been called upon to produce plans, perspectives and models, often at short notice which must have strained his patience and even during the construction period plans and lay-out had been altered and building affected due to high inflation, shortages of bricks because of the three-day week, and strikes.
Mennim's conservation work led to a close association with St Magnus Cathedral in Kirkwall, Orkney and he was responsible for much of its restoration even to the extent of re-opening its original quarry.
Besides many other village interests and activities, he had designed the new Vicarage, served as Chairman of the Parish Council and Churchwarden of the well-known All Hallows Church (where Lawrence Sterne had been Priest) and where he had been married to his wife Eleanor, 53 years earlier.