ˈæftɚ hiː hæd dʒˈɔɪnd ðɪ ˌɪndᵻpˈɛndənt lˈeɪbɚ pˈɑːɹɾi ɪn mˈɜːðɚ, hˈɔːɹnɚ mˈuːvd tə ðə kˈɑːlɪɚɹi vˈɪlɪdʒ ʌv ˈʌnɪʃˌɪɹ ɪnðə ɹˈɑːndə wˌɛɹ hiː bɪkˌeɪm ɐ pɹˈoʊɾəʒˌeɪ ʌv nˈoʊʌ ˈeɪbəlt, tɹˈeɪd jˈuːniən mˈɪlɪtənt, ɛɡzˈɛkjuːtˌɪv mˈɛmbɚɹ ʌvðə sˈaʊθ wˈeɪlz mˈaɪnɚz fˌɛdɚɹˈeɪʃən ænd ˈɔːlsoʊ kənvˈɛnɚɹ ʌv lˈoʊkəl klˈæsᵻz ɪn mˈɑːɹksɪst ˌɛdʒuːkˈeɪʃən wˌɪtʃ hˈɔːɹnɚɹ ɐtˈɛndᵻd.
After he had joined the Independent Labour Party in Merthyr, Horner moved to the colliery village of Ynyshir in the Rhondda where he became a protégé of Noah Ablett, trade union militant, executive member of the South Wales Miners Federation and also convenor of local classes in Marxist education which Horner attended.