nˈoʊ. twˈɛnti wˈʌn skwˈɑːdɹən wʌz fˈɔːɹmd æt ɹˈɑːf stˈeɪʃən lˈævɜːtən ˌɔn twˈɛnti ˈeɪpɹəl nˈaɪntiːnhˈʌndɹɪd θˈɜːɾi sˈɪks, æz ɐ sˈɪɾɪzən ˈɛɹ fˈoːɹs ɹɪzˈɜːv skwˈɑːdɹən.
No. 21 Squadron was formed at RAAF Station Laverton on 20 April 1936, as a Citizen Air Force (reserve) squadron.