- Mackerel sky
- Mad as a hatter
- Mad as a March hare
- Mad dogs and Englishmen go out in the midday sun
- Magical realism
- Main chance
- Make a bee-line for
- Make a clean breast of it
- Make a pig's ear of
- Make a rod for your own back
- Make a virtue of necessity
- Make haste
- Make hay while the sun shines
- Make him an offer he can't refuse
- Make my day
- Make no bones about
- Make your hair stand on end
- Mal de mer
- Man after my own heart
- Man does not live by bread alone
- Man who is his own lawyer has a fool for a client
- man's best friend
- Man's inhumanity to man
- Many a little makes a mickle
- Many are called but few are chosen
- Many happy returns
- Many a true word is spoken in jest
- 'March' phrases
- Mare's nest
- Marital aid
- Market forces
- Marry in haste, repent at leisure
- May you live in interesting times
- McCoy – The real
- Mea culpa
- Meat and drink
- Meat and two veg
- Meet your Waterloo
- Memory lane
- Men in suits
- Mend fences
- Ménage à trois
- Men's evil manners live in brass; their virtues we write in water
- Merry Christmas
- Method in my madness
- Mexican wave
- Mickey Finn
- Middle for diddle
- Middle of the road
- Might and main
- mightier than the sword
- Mighty oaks from little acorns grow
- Milk of human kindness
- Millstone around one's neck
- Minced oaths
- Mince words
- Mind your Ps and Qs
- Mind's ear
- Mind's eye
- Ministering angel shall my sister be
- Mint condition
- Misery acquaints a man with strange bedfellows
- Misquotes
- Miss is as good as a mile
- Moaning Minnie
- Mondegreens
- 'Money' phrases
- Money for old rope
- Monstrous regiment of women
- Montezuma's Revenge
- Monty – The full
- Moot point
- Morbid obesity
- More bang for your buck
- More fool you
- More haste, less speed
- More honoured in the breach than in the observance
- More light!
- Morning person
- Mother country
- Motley crew
- Mouth-watering
- Mouth and no trousers
- Movable feast
- Movers and shakers
- Moving finger writes
- Moving the goalposts
- Much ado about nothing
- Much of a muchness
- Mud
- Mull (something) over
- Multitude of sins
- Mumbo jumbo
- Mum's the word
- Murphy's Law
- Music has charms to soothe the savage breast
- Mutt's nuts
- Mutt and Jeff
- Mutton dressed as lamb
- My bad
- My better half
- My cup of tea
- My cup runneth over
- My giddy aunt
- My husband and I
- My mind's eye
- My kingdom for a horse
- My old china
- My old Dutch
- My salad days
- My stars and garters
- My way or the highway