There are more than 9,500 English words that begin with the letter M. Here are some of the longer ones:

14-letter words

machicolations, machineability, macroaggregate, macroeconomics, macroevolution, macroglobulins, macromolecular, macromolecules, macronucleuses, macronutrients, macrostructure, magnetizations, magnetometries, magnetospheres, magnetospheric, magnifications, magniloquences, magniloquently, maidenlinesses, malabsorptions, malacostracans, maladaptations, maladjustments, maladministers, malapertnesses, malapportioned, malariologists, malcontentedly, malfunctioning, malleabilities, malodorousness, manageableness, manifestations, manifoldnesses, manipulability, manipulatively, mannerlinesses, manometrically, manufacturings, mariculturists, martyrizations, martyrologists, masterlinesses, mastigophorans, materialnesses, mathematically, mathematicians, matriculations, meaningfulness, mechanizations, meddlesomeness, meditativeness, megakaryocytes, megakaryocytic, megalomaniacal, megalopolitans, megascopically, megasporangium, megasporophyll, melodramatised, melodramatises, melodramatists, melodramatized, melodramatizes, memorabilities, mendaciousness, mercantilistic, mercaptopurine, mercerizations, merchandisings, merchandizings, mercifulnesses, meretriciously, mesdemoiselles, mesotheliomata, metafictionist, metalinguistic, metallizations, metallographer, metallographic, metamorphosing, metaphorically, metaphosphates, metaphysically, metaphysicians, metapsychology, metastatically, metathetically, metempsychoses, metempsychosis, meteoriticists, meteorological, meteorologists, methemoglobins, methodicalness, methodological, methodologists, methoxyflurane, methylxanthine, meticulosities, meticulousness, metronidazoles, metronomically, microanatomies, microbarograph, microbiologies, microbiologist, microbreweries, microcassettes, microcephalics, microcephalies, microcircuitry, microcomputers, microeconomics, microelectrode, microevolution, microfibrillar, microfilaments, microgravities, microinjecting, microinjection, micrometeorite, micrometeoroid, microminiature, micronucleuses, micronutrients, microorganisms, microparticles, microplanktons, microprocessor, microprojector, micropublisher, micropulsation, micropunctures, microspherical, microsporangia, microsporocyte, microstructure, microsurgeries, microtechnique, militantnesses, militarization, millenarianism, millennialisms, millennialists, milliroentgens, mindlessnesses, mineralization, miraculousness, mirthfulnesses, misallocations, misapplication, misapprehended, misappropriate, misarticulated, misarticulates, misassumptions, misattributing, misattribution, miscalculating, miscalculation, miscegenations, mischannelling, misclassifying, miscomputation, misconceptions, misconnections, miscorrelation, misdescription, misemphasizing, misemployments, misestimations, misevaluations, misfunctioning, misgovernments, misidentifying, misimpressions, misinformation, misinterpreted, mismanagements, misorientation, misperceptions, mispositioning, misprogramming, mispronouncing, misregistering, misremembering, misrepresented, missionization, mistranscribed, mistranscribes, mistranslating, mistranslation, misunderstands, misutilization, mitogenicities, moderatenesses, moderatorships, modernisations, modernizations, modulabilities, mollifications, moneygrubbings, mongrelization, monocarboxylic, monochromatism, monochromators, monocotyledons, monoglycerides, monolithically, monomaniacally, monometallisms, monometallists, mononucleotide, monophonically, monopolization, monopropellant, monosaccharide, monotheistical, monotonicities, monotonousness, monounsaturate, monumentalized, monumentalizes, moralistically, morganatically, mortifications, motherlessness, motherlinesses, motionlessness, motivationally, mountaineering, mournfulnesses, movelessnesses, mucilaginously, muddleheadedly, mulligatawnies, multichambered, multicharacter, multicomponent, multiconductor, multidialectal, multielectrode, multifactorial, multifilaments, multiformities, multifrequency, multilaterally, multilingually, multimegawatts, multimolecular, multinationals, multinucleated, multiparameter, multiplication, multiplicative, multiplicities, multipotential, multiprocessor, multiracialism, multireligious, multitrillions, multiversities, mummifications, municipalities, municipalizing, musicalization, mutagenicities, mutinousnesses, mutualizations, myelencephalic, myelencephalon, myrmecological, myrmecologists, myrmecophilous, mysteriousness, mystifications, mythologically

15-letter words

machinabilities, macroaggregated, macroaggregates, macrocosmically, macroevolutions, macrophotograph, macroscopically, macrostructural, macrostructures, magistratically, magnanimousness, magnetoelectric, maintainability, majoritarianism, maladministered, maladroitnesses, maldistribution, maliciousnesses, malpractitioner, manageabilities, maneuverability, marginalization, marketabilities, marriageability, martensitically, marvelousnesses, masculinization, masochistically, masterfulnesses, mastoidectomies, materfamiliases, materialization, mathematization, meaninglessness, measurabilities, mechanistically, mechanochemical, mechanoreceptor, medulloblastoma, megacorporation, megagametophyte, megasporophylls, mellifluousness, melodiousnesses, melodramatising, melodramatizing, memorablenesses, mensurabilities, mercaptopurines, mercenarinesses, merchantability, mercilessnesses, mercurialnesses, meritoriousness, meroblastically, metafictionists, metalinguistics, metallographers, metallographies, metallurgically, metamathematics, metamorphically, metastabilities, methamphetamine, methoxyfluranes, methylcellulose, methylmercuries, methylphenidate, methylxanthines, microanalytical, microanatomical, microbarographs, microbiological, microbiologists, microcosmically, microdissection, microearthquake, microelectrodes, microelectronic, microevolutions, microgametocyte, microinjections, micromanagement, micrometeorites, micrometeoritic, micrometeoroids, micromorphology, microphotograph, microphotometer, microphotometry, microphysically, microporosities, microprocessors, microprojection, microprojectors, micropublishers, micropublishing, micropulsations, microradiograph, microscopically, microseismicity, microsporangium, microsporocytes, microsporophyll, microstructural, microstructures, microtechniques, microtonalities, militarizations, millenarianisms, millionairesses, mineralizations, mineralogically, miniaturization, misapplications, misapprehending, misapprehension, misappropriated, misappropriates, misarticulating, misattributions, miscalculations, miscegenational, miscellaneously, mischaracterize, mischievousness, miscomputations, misconstruction, miscorrelations, misdescriptions, misdistribution, miserablenesses, misguidednesses, misinformations, misinterpreting, misorientations, misrecollection, misregistration, misrepresenting, missionizations, mistranscribing, mistranslations, mistrustfulness, misutilizations, misvocalization, modifiabilities, momentarinesses, momentousnesses, mongrelizations, monochromatisms, monocrystalline, monomolecularly, mononucleosises, mononucleotides, monopolizations, monopropellants, monosaccharides, monospecificity, monosyllabicity, monounsaturated, monounsaturates, monstrousnesses, montmorillonite, monumentalities, monumentalizing, morphologically, morphophonemics, mountaineerings, mountainousness, mountebankeries, mouthwateringly, multidiscipline, multidivisional, multifunctional, multilateralism, multilateralist, multilingualism, multimillennial, multiplications, multipolarities, multiprocessing, multiprocessors, multiracialisms, multitudinously, multiwavelength, murderousnesses, musculoskeletal, musicalizations, myelofibrosises

16-letter words

machineabilities, macroinstruction, macrolepidoptera, macrophotographs, macrophotography, magnetostriction, magnetostrictive, majoritarianisms, maladministering, malapportionment, malcontentedness, maldistributions, malodorousnesses, malpractitioners, manageablenesses, manipulabilities, manipulativeness, marginalizations, masculinizations, materializations, mathematizations, meaningfulnesses, mechanochemistry, mechanoreception, mechanoreceptive, mechanoreceptors, meddlesomenesses, meditativenesses, medulloblastomas, megacorporations, megagametophytes, megalomaniacally, megasporogeneses, megasporogenesis, melodramatically, mendaciousnesses, meretriciousness, meristematically, mesembryanthemum, metamathematical, metapsychologies, meteorologically, methamphetamines, methodicalnesses, methodologically, methylcelluloses, methylphenidates, meticulousnesses, microcalorimeter, microcalorimetry, microcircuitries, microcirculation, microcirculatory, microcrystalline, microdissections, microearthquakes, microelectronics, microencapsulate, microenvironment, microgametocytes, micrographically, microinstruction, microlepidoptera, micromanagements, micromanipulator, micrometeorology, microphotographs, microphotography, microphotometers, microphotometric, microprogramming, microprojections, micropublishings, microradiographs, microradiography, microsporangiate, microsporophylls, microvasculature, militaristically, miniaturizations, miraculousnesses, misanthropically, misapprehensions, misappropriating, misappropriation, mischaracterized, mischaracterizes, miscommunication, miscomprehension, misconstructions, misdistributions, mispronunciation, misrecollections, misregistrations, mistranscription, misunderstanding, misvocalizations, monochromaticity, monocotyledonous, monopolistically, monosyllabically, monosynaptically, monotheistically, monotonousnesses, montmorillonites, montmorillonitic, morphometrically, motherlessnesses, motionlessnesses, muddleheadedness, multibillionaire, multicellularity, multiculturalism, multidimensional, multidirectional, multidisciplines, multifactorially, multifariousness, multihandicapped, multilateralisms, multilateralists, multilingualisms, multimillionaire, multiplicatively, multiprocessings, multiprogramming, municipalization, mysteriousnesses

17-letter words

macroevolutionary, macroglobulinemia, macroglobulinemic, macroinstructions, magnanimousnesses, magnetoresistance, magnetostrictions, maintainabilities, maladministration, malapportionments, maneuverabilities, marriageabilities, materialistically, meaninglessnesses, mechanoreceptions, medulloblastomata, mellifluousnesses, melodramatization, merchantabilities, meritoriousnesses, mesembryanthemums, metapsychological, methemoglobinemia, methylnaphthalene, microbiologically, microcalorimeters, microcalorimetric, microcirculations, microdensitometer, microdensitometry, microencapsulated, microencapsulates, microenvironments, microevolutionary, microinstructions, micromanipulation, micromanipulators, microminiaturized, micromorphologies, micropaleontology, microphotographed, microphotographer, microphotographic, microphotometries, microprogrammings, microradiographic, microreproduction, microseismicities, microsporogeneses, microsporogenesis, microvasculatures, mineralocorticoid, misadministration, misappropriations, miscellaneousness, mischaracterizing, mischievousnesses, misclassification, miscommunications, miscomprehensions, misidentification, misinterpretation, mispronunciations, misrepresentation, misrepresentative, mistranscriptions, mistrustfulnesses, misunderstandings, monochromatically, monospecificities, monosyllabicities, morphogenetically, mountainousnesses, multibillionaires, multiculturalisms, multidisciplinary, multigenerational, multimillionaires, multiprogrammings, multitudinousness, municipalizations

18-letter words

macroglobulinemias, macrophotographies, magnetoresistances, magnetostrictively, maladministrations, malcontentednesses, manipulativenesses, mechanochemistries, melodramatizations, meretriciousnesses, metallographically, methemoglobinemias, methylcholanthrene, methylnaphthalenes, methylprednisolone, microcalorimetries, microcrystallinity, microdensitometers, microdensitometric, microencapsulating, microencapsulation, microenvironmental, microlepidopterous, micromanipulations, micrometeorologies, micrometeorologist, micromorphological, micropaleontologic, microphotographers, microphotographies, microphotographing, microradiographies, microreproductions, mineralocorticoids, misadministrations, misclassifications, misidentifications, misinterpretations, misrepresentations, monochromaticities, mucopolysaccharide, muddleheadednesses, multicellularities, multifariousnesses, myeloproliferative

19-letter words

magnetohydrodynamic, meningoencephalitic, meningoencephalitis, methylcholanthrenes, methylprednisolones, microdensitometries, microelectronically, microencapsulations, micrometeorological, micrometeorologists, micropaleontologies, micropaleontologist, miscellaneousnesses, mischaracterization, mucopolysaccharides, multidimensionality, multitudinousnesses

20-letter words

magnetofluiddynamics, magnetohydrodynamics, microcrystallinities, microelectrophoreses, microelectrophoresis, microelectrophoretic, microminiaturization, micropaleontological, micropaleontologists, microphotometrically, mischaracterizations

21-letter words

meningoencephalitides, microminiaturizations, multidimensionalities

22-letter words

microspectrophotometer, microspectrophotometry

23-letter words

microspectrophotometers, microspectrophotometric

24-letter words

microelectrophoretically, microspectrophotometries