The county seat is about 80 km south of Poznań. The church is in ul. Kościelna in the centre of the city.
St. Nicholas Collegiate Church is Leszno’s main place of worship. The Roman Catholic St. Nicholas parish has been here a century and a half longer than the city of Leszno. Intitially, a small brick Gothic church stood here. This was transferred to the Unity of the Brethren after 1580. This church was destroyed by a fire in the city in 1656. The Catholics regained their temple in 1661 but declined to rebuild it due to the extent of the damage and the lack of funds.
Bp. Bogusław II Leszczyński funded a completely new temple, designed by Giovanni Catenazzi, after 1680. The building was completed in 1719. A new presbytery was added to a plan by Arnold Güldenpfennig from Paderborn in 1905-1907.
Until 1939, St. Nicholas Church was the only Catholic place of worship in the city. During the Nazi occupation, the church was available to Polish people until October 1941 when it served German Catholics.
The roof of the temple was damaged by artillery shells in 1945. Conservation works were carried out in the 1950s and again in the early 1980s. The temple was elevated to collegiate status in 2000. The interior of the church was thoroughly renovated in 2007.
The nave of St. Nicholas Church has preserved its baroque decor and the presbytery its neo-baroque decor. The high altar dates from 1744 and has a painting by Aleksander Józef Sułkowski. The central panel has a painting of the Holy Trinity being worshipped by the Virgin Mary (Michelangelo Palloni). The pulpit is an outstanding work of woodcarving artistry made in the 1st half of the 18th century, probably in Silesia. A stucco sculpture of St. John of Nepomuk stands opposite this superb artistic form. The epitaph of the founders of the temple, Bogusław (d. 1691) and Rafał (d. 1703) Leszczyński, is one of the most beautiful baroque funerary monuments in Poland.
The church has 10 side altars from the late 17th century to the 1st half of the 18th. The Leszczyński family’s Wieniewa coat of arms hangs on the front of the choir organ. A marble pieta from a demolished chapel in Duszniki, Lower Silesia, was installed by the main entrance to the Chapel of Perpetual Adoration in 1973. The southern exterior wall of the church has epitaph plaques from the 17-19th centuries and commemorative plaques from the 20th.