A mansionary (from the Latin mansio, ‘a dwelling’) was a member of a community of four to ten secular priests governed by a provost and required to reside by and serve a chantry chapel, similar to a cathedral canon or beneficed chantry priest. Every day they would sing the Hours of Our Lady and offer two Masses, one in honour of Our Lady or the Holy Trinity, and the other for the dead kin of the chantry founder. The chapels they served were attached to a cathedral or a parish church. Those established by the monarch often had pastoral duties, sometimes involving a school or hospice. In Lithuania, they appear from the late 15th century at the cathedrals of Vilnius, Varniai and Lutsk (in Janów Podlaski), and represented a considerable financial investment to establish and maintain. After the Council of Trent, they become even rarer, and concentrate more on pastoral and other educational duties. The paper discusses what a mansionary priest was, and how many of them served in the Diocese of Vilnius and other sees within the Grand Duchy of Lithuania. Why was it deemed meet and fit to establish a mansionariate in Lithuania at the turn of the 15th and 16th centuries, and how were such foundations affected by local Reform movements?