The Roman Catholic Community of Merced began in Mariposa in 1867. Merced was a mission of St. Joseph’s Parish in Mariposa. In 1873 the first baptisms and initial Masses in Merced were at the newly constructed home of Mathias Smythe, located on the southeast corner of 19 and L Streets. Later masses were held at the Washington Hall on 17 Street. In 1873 Father McNamara had raised enough capital to purchase church property at 21 and L Streets. The church was dedicated to St. Patrick and a week later the church was formally blessed and dedicated by Archbishop Alemany. On April 28, 1918 the new church was completed and the name changed from St. Patrick to Our Lady of Mercy. Vatican records still list the parish officially as St. Patrick's Parish, and the church itself as Our Lady of Mercy.
In 1971, Bishop Donohoe initiated the Pastoral Team, combining the Merced churches into one parish (Our Lady of Mercy, St. Patrick’s and Sacred Heart). In 1981, a committee was formed to look at beginning a process to build a much larger church for St. Patrick’s. The community had been meeting at Our Lady of Mercy School McKenna Hall for Mass on Sundays. In 1989 the team ministry ended and Our Lady of Mercy and St. Patrick’s were combined to be one parish. In 1991 Father David A. Brum began a final fund-raising campaign towards the new St. Patrick’s Church. The property of the new church was blessed on Palm Sunday, April 1993. On July 1, 1993, Father Jean-Michael Lastiri was assigned as Pastor and finalized the plans with ground breaking on November 12, 1993, by Bishop John Steinbock. Construction began immediately and the church was completed in December of 1994. In 1997, the parish was able to purchase the Love property that was directly north of the new St. Patrick’s Church. This parcel of 61/2 was added to the existing nine acres, given years earlier by Spalding Wathen for the new church. On the property were an old home, barn, and other vestiges of an old dairy that once operated at the site. After over $100,000 was spent in bringing the former Love residence to compliance, the edifice was named in honor of Monsignor John Harguindeguy, former pastor of Our Lady of Mercy/St. Patrick’s, who served in Merced for many years before moving on to other assignments in Fresno. It was formally dedicated and blessed by Bishop John T. Steinbock on March 17, 1998.
As the parish continued to grow, needs began to emerge, calling for a social hall, additional classrooms, and eventually a new office for the parish. After meetings and discussions with the Finance Council and Parish Pastoral Council, a decision was made to engage a development company to handle the Capital Campaign to build such a building. Craig Scott of DuPertuis and Scott, architects of St. Patrick’s Church was hired as architect, and plans were drawn up for the new structure. The campaign began in June 1999. The construction was to be in four phases. The first would be the new parking lot north of the church, expanding Yosemite Avenue frontage, and installing a fire land and new lights, in compliance with city codes before construction of the hall. The second phase is the new parish center that includes hall, which will seat 1100 or 500 for dining, classroom with bathroom, large restrooms, small formal dining room, and large storage spaces. The third phase will be the future offices of the parish, and the last phase of the overall project will be the small daily Mass chapel that will be attached to the parish center. The Harguindeguy Center now serves as the parish offices, with a full staff on site. A new small park has also be added for outdoor parish activities. The new parish center is now completed and dedicated on November 23, 2002 by Bishop John T. Steinbock.