Hoser was trained as a physician before joining the Pallottines in 1969.

He was president of the Pontifical Mission Societies before he was elevated to the title of archbishop ad personam in 2005 by Pope John Paul II. On 24 May 2008, Pope Benedict XVI appointed him to head Roman Catholic Diocese of Warszawa-Praga. He was a member of the Presidium of the Polish Episcopal Conference and headed the Team of Experts on Bioethics. Bishop Hoser was a staunch opponent of in vitro fertilisation.

Hoser was intensely criticised in 2013, when he refused to remove a priest from active duty who was convicted of molesting two altar boys. On the other hand, Hoser removed a priest Wojciech Lemanski, who used his pulpit to continually and vocally criticize the church. Lemanski’s criticisms centered around anti-Semitic tendencies in Poland; the church’s lenient treatment of clerics accused of sexual abuse; the church’s opposition to artificial insemination and contraceptives.

On 11 February 2017, Pope Francis named Bishop Hoser his special envoy to Medjugorje, tasked with assessing the pastoral needs of that place of pilgrimage.

Archbishop Hoser delivered Pope Francis his examination of the pastoral situation of Medjugorje in the summer of 2017. He was explicitly tasked by the Pope to assess the pastoral situation and not the doctrinal situation which was done by Cardinal Ruini. However he made it clear that he thought very highly of the Medjugorje phenomenon. After his resignation as archbishop of the diocese of Warszawa-Praga on 8 December 2017 he was made Apostolic Visitor for an undetermined time for Medjugorje by Pope Francis on 31 May 2018.

Hoser died from COVID-19 on August 13, 2021, during the COVID-19 pandemic in Poland.