Hannigan ta fara fitowa a cikin tallace-tallace a lokacin da take da shekaru hudu. Ta koma Hollywood a lokacin da take da shekaru 11. Ta zauna tare da mahaifiyarta kuma ta halarci Makarantar Sakandaren Arewacin Hollywood, ta yi nasarar yin gwaji ga wakilai yayin da ta ziyarci mahaifinta a Santa Barbara. Bayan halartar Makarantar Sakandaren Arewacin Hollywood, ta halarci Jami'ar Jihar California, Northridge, inda ta kasance memba na Alpha Chi Omega kuma ta sami digiri a fannin ilimin halayyar dan adam.Babban rawar fim na Hannigan shine a cikin My Stepmother Is an Alien, wani wasan kwaikwayo na almara na kimiyya da aka saki a shekarar 1988; daya daga cikin 'yan wasan kwaikwayo a cikin fim din shi ne Seth Green, wanda daga baya ya shiga ta a cikin Buffy na yau da kullum a matsayin saurayinta a kan allo.
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