Zygmunt Mycielski supported the anti-communist opposition in Poland. Although he himself did not join the Workers’ Defence Committee, he often spoke against the government’s policy. In addition, he maintained contact with opposition activists, especially Adam Michnik. He met him, when Michnik was Antoni Słonimski’s secretary and quickly took a liking to the man. In the 1980s Mycielski was worried about Michnik and his colleagues; he wrote letters to the prison where Michnik was kept. Adam Michnik was moved, when he recalled his acquaintance with Zygmunt Mycielski. As he said in 2016:

There were certainly writers and composers greater than him – and he was well aware of that. And yet Zygmunt was someone more – a mere conversation with him was something extraordinary and incredibly life-changing. (Szlachectwo zobowiązuje, Kraków 2018).