In the early part of the 19th century, the fledgling Catholic community of Leominster was served by clergy from Saint Bernard's Church, Fitchburg, and by the Bishop of Boston. Masses were celebrated in homes and public facilities until 1851 when a little white church at the corner of Main and Priest Streets was purchased from a Baptist congregation.

Then, in 1872, the sixty-five Catholic families of Leominster were finally given independent status from Saint Bernard's Parish and Saint Leo's Parish was established as the first permanent Catholic parish in the city.

A new church was purchased from the Methodists right in downtown Leominster at 91 Main Street. The first permanent pastor arrived in 1872 in the person of Reverend Daniel Sheil. He was sent by the Bishop of Springfield, the Most Reverend Thomas Beaven who had established that diocese in 1870.

1900 was a momentous year for Saint Leo's. Two very significant events occurred: first, the present church was completed at 128 Main Street and, secondly, the French community split off from Saint Leo's and Saint Cecilia's Parish was formed.

From 1901-1905, the beautiful Tiffany-style stained glass windows were installed and, in 1913, the present rectory and carriage house was purchased from the Phelps Estate.
In 1925, Saint Leo's School opened and it was staffed by the Sisters of the Presentation of the Blessed Virgin Mary in Fitchburg. Their convent was completed in 1930.

In 1937 the Italian community formed Saint Anna's Parish and, in 1948, a six-room addition was added to the school and a Casavant pipe organ installed in the church. In 1950, the Diocese of Worcester was created and Saint Leo's now came under the Bishop of Worcester.

In 1952 Our Lady of the Lake Parish was formed from the North Leominster Saint Leo's parishioners and in 1954 the auditorium building was completed. In 1963 the fourth parish formed from the mother church of Leominster was established and dedicated to the Holy Family.

In 2022 St. Leo's celebrated its 150th anniversary.  St. Leo's has been a parish community celebrating its faith in Jesus Christ, and because of that faith, has been serivng the needs of parishioners and the greater community for 150 years.