In 1903, the Diocese of Pittsburgh established 13 new Catholic parishes. Of those parishes, only three remain as they were founded. Assumption is one of those three parishes. In the years just prior to 1900, Bellevue, Pennsylvania, began to offer choice home sites with yards and gardens to entice people from the more crowded city neighborhoods of Old Allegheny (now known as the North Side) and the City of Pittsburgh. The Catholics who moved out of the City attended Mass at either St. Francis Xavier Church on California Avenue, or Sacred Heart in Emsworth. By 1903, this small band of pioneers discussed founding their own parish. Bishop Richard Phelan of the Pittsburgh diocese agreed and sent Father Israel J. McGovern, assistant at St. Mary’s in Lawrenceville, to establish a parish in Bellevue.
The first Mass was celebrated in Greenough Hall, a meeting and dance hall located above a livery stable on Rogers Avenue, now Balph Avenue (the present site of Classic Chevrolet). The United Presbyterian Church loaned chairs and the altar was borrowed. On that day, there was a smallpox scare in Avalon, and the Bellevue authorities posted guards at the borough lines to prevent people from entering from Avalon. Not to be denied the privilege of attending the first Mass, Avalon residents walked back a few streets, boarded a streetcar, and rode past the guards to Mass. At 8 a.m. on Sunday morning, September 13, 1903, with 75 people in attendance at the first Mass, the parish community of the Assumption of the Blessed Virgin Mary on the Beautiful River was born.