Long words (12 or more letters) that start with M are shown in the list below.
macadamizing, machinations, mademoiselle
mademoiselles, magestically, magistracies
magnanimities, magnanimously, magnanimousness
magnanimousnesses, magnetically, magnetizable
magnetization, magnetizations, magnification
magnifications, magnificence, magnificences
magnificently, maintainabilities, maintainability
maintainable, maintainance, maintainances
maintenances, maladjustment, maladjustments
malapropisms, maledictions, maleficences
malevolences, malfeasances, malformation
malformations, malfunctioned, malfunctioning
malfunctions, malignancies, malleabilities
malleability, malnourished, malnutrition
malnutritions, malodorously, malodorousness
malodorousnesses, malpractices, maltreatment
maltreatments, manageabilities, manageability
manageableness, manageablenesses, managemental
maneuverabilities, maneuverability, manifestation
manifestations, manipulating, manipulation
manipulations, manipulative, manipulators
mannerliness, mannerlinesses, mannishnesses
manorialisms, manslaughter, manslaughters
manufactured, manufacturer, manufacturers
manufactures, manufacturing, marchionesses
marketplaces, marksmanship, marksmanships
marriageable, marshmallows, marvelousness
marvelousnesses, masculinities, masculinization
masculinizations, masqueraders, masquerading
massachusetts, massivenesses, masslessness
masslessnesses, mastectomies, mastications
masturbating, masturbation, masturbations
materialisms, materialistic, materialists
materialization, materializations, materialized
materializes, materializing, mathematical
mathematically, mathematician, mathematicians
matriarchies, matriculated, matriculates
matriculating, matriculation, matriculations
matrimonially, maturational, mawkishnesses
meagernesses, meaningfully, measurements
mechanically, mechanistically, mechanization
mechanizations, medievalisms, medievalists
mediocrities, meditatively, meetinghouse
meetinghouses, melancholies, meliorations
mellifluously, mellifluousness, mellifluousnesses
mellownesses, melodiousness, melodiousnesses
melodramatic, melodramatist, melodramatists
memorabilities, memorability, memorableness
memorablenesses, memorialized, memorializes
memorializing, memorization, memorizations
mendaciously, mendicancies, meningitides
menstruating, menstruation, menstruations
mercenariness, mercenarinesses, merchandised
merchandiser, merchandisers, merchandises
merchandising, mercurialness, mercurialnesses
meritoriously, meritoriousness, meritoriousnesses
merrymakings, mesdemoiselles, metabolizing
metallurgical, metallurgically, metallurgies
metallurgist, metallurgists, metalworkers
metalworking, metalworkings, metamorphose
metamorphosed, metamorphoses, metamorphosing
metamorphosis, metaphorical, metaphysical
metaphysician, metaphysicians, meteorically
meteorological, meteorologies, meteorologist
meteorologists, methodically, methodicalness
methodicalnesses, methodological, methodologies
methotrexate, meticulously, meticulousness
meticulousnesses, metrications, metropolises
metropolitan, microbiological, microbiologies
microbiologist, microbiologists, microbiology
microcomputer, microcomputers, microfilming
microminiature, microminiatures, microminiaturization
microminiaturizations, microminiaturized, microorganism
microorganisms, microscopical, microscopically
microscopies, migratational, migratations
militaristic, millionaires, mimeographed
mimeographing, mindlessness, mindlessnesses
mineralizing, minerological, minerologies
minerologist, minerologists, miniaturists
miniaturized, miniaturizes, miniaturizing
minicalculator, minicalculators, minicomponent
minicomponents, minicomputer, minicomputers
miniconvention, miniconventions, minicrisises
minifestival, minifestivals, minihospital
minihospitals, minilectures, minimiracles
minimization, mininetworks, miniproblems
minirebellion, minirebellions, minirecession
minirecessions, miniscandals, miniserieses
minisocieties, ministration, ministrations
minisubmarine, minisubmarines, miniterritories
miniterritory, minitheaters, minivacation
minivacations, miniversions, minstrelsies
minutenesses, miraculously, mirthfulness
mirthfulnesses, misanthropes, misanthropic
misanthropies, misapprehend, misapprehended
misapprehending, misapprehends, misapprehension
misapprehensions, misappropriate, misappropriated
misappropriates, misappropriating, misappropriation
misappropriations, misbeginning, misbehaviors
miscalculate, miscalculated, miscalculates
miscalculating, miscalculation, miscalculations
miscarriages, miscegenation, miscegenations
miscellaneous, miscellaneously, miscellaneousness
miscellaneousnesses, miscellanies, mischievously
mischievousness, mischievousnesses, misconceived
misconceives, misconceiving, misconception
misconceptions, misconstruction, misconstructions
misconstrued, misconstrues, misconstruing
misdemeanors, misenrolling, miserableness
miserablenesses, miserlinesses, misinferring
misinformation, misinformations, misinterpret
misinterpretation, misinterpretations, misinterpreted
misinterpreting, misinterprets, misinterring
misjudgments, mislabelling, misleadingly
mismanagement, mismanagements, mispronounce
mispronounced, mispronounces, mispronouncing
mispronunciation, mispronunciations, misquotation
misquotations, misreferring, misrepresent
misrepresentation, misrepresentations, misrepresented
misrepresenting, misrepresents, missionaries
misspellings, misstatement, misstatements
mistreatment, mistreatments, mistrustfully
mistrustfulness, mistrustfulnesses, misunderstand
misunderstanded, misunderstanding, misunderstandings
misunderstands, mobilization, mobilizations
mockingbirds, moderateness, moderatenesses
modernization, modernizations, modernnesses
modification, modifications, modularities
molestations, mollification, mollifications
mollycoddled, mollycoddles, mollycoddling
momentousment, momentousments, momentousness
momentousnesses, monastically, monasticisms
moneylenders, monkeyshines, monkishnesses
monogramming, monologuists, mononucleosis
mononucleosises, monopolistic, monopolization
monopolizations, monopolizing, monosyllable
monosyllables, monosyllablic, monotonously
monotonousness, monotonousnesses, monstrosities
monumentally, moonlighters, moonlighting
morbidnesses, moribundities, morosenesses
morphologically, morphologies, mortification
mortifications, motionlessly, motionlessness
motionlessnesses, motorcyclist, motorcyclists
mountaineered, mountaineering, mountaineers
mountaintops, mournfullest, mournfulness
mournfulnesses, mucilaginous, mulishnesses
multibarreled, multibillion, multibranched
multibuilding, multichambered, multichannel
multicolored, multicultural, multidenominational
multidimensional, multidirectional, multidisciplinary
multidiscipline, multidivisional, multidwelling
multifaceted, multifarously, multifilament
multifunction, multifunctional, multihospital
multilateral, multilingual, multilingualism
multilingualisms, multimillion, multimillionaire
multimodalities, multimodality, multipartite
multiplexors, multiplication, multiplications
multiplicities, multiplicity, multiproblem
multiproduct, multipurpose, multiservice
multisyllabic, multitalented, multitudinous
multivariate, multiwarhead, mummification
mummifications, municipalities, municipality
munificences, muscularities, musculatures
musicianship, musicianships, mutabilities
myelosuppression, myelosuppressions, mysteriously
mysteriousness, mysteriousnesses, mystification
mystifications, mythological, mythologists
Help make this word list more complete by adding additional words here in the comments. Thanks!