After his ordination in 1882 or 1885 in Florence, he was sent on a diplomatic mission to Spain, and a year later for the first time went to the East Indies, accompanying Antonio Agliardi, the Titular Archbishop of Cesarea and first Apostolic Delegate in India.

For a time, Zaleski remained employed in the Roman Curia, as consulter on Eastern affairs at the Congregation for the Propagation of the Faith (Propaganda Fidei).

In 1890 he returned to India, where on 5 March 1892 he replaced Archbishop Andrea Aiuti as the Apostolic Delegate of the East Indies.

The area of his official activities ranged from Afghanistan and the Himalayas in the north, to Ceylon and the neighboring islands of the Indian Ocean in the south.

[2] Zaleski also initiated the creation of many minor seminaries and ordained several bishops, including local priests Augustine Kandathil[3] (1911) and Alexander Chulaparambil (1914).

During his tenure as the Apostolic Delegate, he broadened his polyglot skills having already mastered eight European languages, to which he added Tamil and Sinhalese.

Archbishop Zalesky was Apostolic Delegate for the supervision of all Roman Catholic missions of the East Indies, that is, over an area that was about half as large as the whole of Europe, including Russia and in which 300 million people lived.

He learned in this way about the whole of India, other countries, peoples and states, their kings and princes, the different races and religions, the character of the individual tribes.

"[4]Ladislaus Zaleski presided over several provincial synods in India, and worked to strengthen the local church hierarchy and promote sustainable growth of the missions.

Pope Benedict considered in 1919 his candidacy for the nomination of Cardinal, but eventually in the consistory that year other Polish bishops received the scarlet – Edmund Dalbor and Alexander Kakowski.