Immaculate Conception was founded in 1855. The origin of the parish can be traced back to 1831 when the first Mass was celebrated in the community. For the next three years, the community was periodically visited by a nearby priest. In 1834 the bishop in Philadelphia made Washington a mission station of St. Peter in Brownsville. When St. James, West Alexander received a resident pastor in 1852, responsibility for Washington was transferred to that parish. Shortly after this, work began on a church in Washington.
The church was completed and dedicated on August 26, 1855. The pastor of St. James then took residence in Washington and St. James became a mission of Immaculate Conception. The church was damaged by a fire in 1885. The building was repaired and continued to be used by the parish for another 7 years. The rising population made a larger church necessary. This new church was dedicated in 1892.
This church, in turn, was damaged in a fire in 1924. While the church was being repaired, Mass was celebrated in the school auditorium. Although the church was repaired and the parish resumed services in it, the continually growing size of the congregation made a new church necessary. Construction of the new church began in 1929 and the completed building was dedicated on September 1, 1930. The old church was sold on October 1, 1946. The stained glass windows of the church were installed in 1961.