On 14 December 1941, he was moved to Dachau and assigned to the priests’ block. Under the harsh conditions of the camp his TB worsened and his hopes of being ordained a priest seemed unachievable. Then, as Providence would have it, Bishop Gabriel Piguet of Clermont-Ferrand arrived in Dachau as a fellow-prisoner on 6 September 1944 – and only a bishop is authorised to confer the sacrament of ordination. This was duly requested for Leisner by a Belgian priest, Fr de Coninck.
Bishop Piguet agreed, on condition that the ordination was authorised by the bishop with whom Leisner was affiliated and also that of the Archbishop of Munich, as Dachau was in his diocese. These authorisations were obtained clandestinely through the good offices of a young woman, Josefa Imma Mack (she was later to become a nun). She used to visit the plant shop at the edge of the compound at Dachau, where flowers and food grown by the prisoners was sold to the public, and where she was able to communicate with priest-prisoners assigned to work there.
She smuggled in the necessary letters of approval, along with the holy oil, a stole and the ritual books. The ordination, on 17 December 1944, took place in the chapel in Block 26 in great secrecy. Zeller records that the ceremony “made a lasting impression on the priests who were present.” The Bishop’s violet cassock was made from fabric from stocks pillaged by the Germans; the mitre was made by Fr Albert Durand, the only British priest at Dachau. Several hundred clergymen supported the young deacon, who wore an alb over his striped prison clothes.