On 19 July 1868, De Waal became chaplain of the German National church of Santa Maria dell'Anima in Rome. He received his Doctorate in Theology on 19 February 1869. During the siege and conquest of Rome by army of the Kingdom of Italy in 1870, he volunteered as chaplain of the Papal troops. He became Vice-Rector in 1872 and then Rector in 1873 of the Collegio Teutonico in the Vatican. In 1875, he was appointed secret chamberlain of Pope Pius IX. On 23 June 1896 Anton de Waal became prelate to the Pontifical House and on 30 July 1900 Protonotary apostolic. He was appointed on 2 November 1904 Commissioner of the pastoral care of all Germans in Italy.