Robert John Mueller was born on 9 September 1922 in Madison, Wisconsin to Matthias P. and Augusta B. (Bauer) Mueller. Father Mueller was the second oldest of six children namely, James M. Mueller, Ardythe E. Griffin, Howard P. Mueller, Patricia M. Marx and Marilyn J. Schneider. Father Mueller attended Blessed Sacrament Grade School and was taught by the Sinsinawa Dominican Sisters. He then went to West High School in Madison and later transferred and graduated from De Pere High School in De Pere, Wisconsin. After high school he studied for one year at Northland College in Ashland, Wisconsin before responding to a call to religious life. He entered the novitiate for the Dominican Order on 13 September 1942 at Saint Thomas Aquinas Priory, River Forest, Illinois and was given the religious name Matthias. He professed his first vows on 14 September 1943. He began his studies for the priesthood at the Pontifical Institute of Philosophy (Dominican House of Studies) in River Forest, Illinois and the Studium Generale Santo Tomas in Avila, Spain. On 14 September 1946 he professed his solemn vows at Saint Thomas Aquinas Priory and was ordained to the priesthood on 10 April 1949 in Avila, Spain by Auxiliary Bishop Martinez of the Diocese of Toledo, Spain. He furthered his studies and earned a Ph.D. in Social Philosophy from the Angelicum in Rome in 1956.

The early years of ministry in his priestly life were as a teacher and associate pastor. From 1950-53 he taught at Edgewood College as well as helped at Blessed Sacrament Parish in Madison, Wisconsin. From 1953-55 he taught Philosophy and Theology at Saint Mary of the Plains College in Dodge City, Kansas. He was Master of Cooperator Brothers and Novices at the Dominican House of Studies and an instructor at Rosary College in River Forest from 1956-57. In 1958 he was assigned to our mission in Bolivia and taught Philosophy in the major seminary in La Paz, Bolivia and later was professor of Philosophy and Theology at the Catholic Normal School (Teacher's College). For two years, 1966-68 he worked in adult education and was the administrator of the Social Center of John XIII in Oruro, Bolivia. From 1968-71 he was a priest worker at a local brewery in Santa Cruz, Bolivia and helped families with obtaining or improving electricity in their homes. From 1971-76 he was the administrator of the Orientation and Resettlement Program for new settlers in San Juan de Piray Colony North of Santa Cruz. He was called into leadership for the brothers in the vicariate of Bolivia and served as the Vicar Provincial from 1979-87. During this time, he also worked in evangelization of peasant farmers in the area. He returned to the Province in the United States in 1987 in order to care for his aging and infirmed mother in Janesville, Wisconsin. During that time, he also served as associate pastor at Saint Pius V Parish until several months before his death. During his time at Saint Pius V, he lived with other priests on the parish staff at the Carlos Morales Community, located across the street from Saint Pius V priory.

Father Matthias touched many people during his years of Saint Pius V. He heard confessions almost every Saturday for 30 years. He officiated at the 7:45 AM Sunday Mass almost on a weekly basis. Those attending the daily 8:00 AM Mass grew to love and care for him. He was known for his quiet and gentle manner with people and his compassion for the

poor. His homilies always connected the scriptures with the real world. He constantly included reflections on injustice, such as racism, income inequality, sexism, war, death penalty, and discrimination against immigrants, and challenged the parish to respond with the forcefulness of Jesus Christ. Many homeless men got to know him because he closed the Church most nights. When it was freezing outside, he allowed them to remain in church overnight. He presided at many wakes, quincea eras and weddings. And for many years he walked through the neighborhood on Sunday mornings after Mass visiting the sick and taking them Communion. For many years, he wrote the bulletin reflection on the Sunday's readings and for most of his years at the parish wrote the Prayers of the Faithful.

Father Matthias was also known for his love of nature. He was a bird watcher and every September spent his vacation camping in a tent in Wisconsin state parks. He loved being close to nature and the solitude it provided him.

In the Spring of 2019, Father Matthias injured himself in a fall in the apartment he lived in across the street from Saint Pius V Church. He was hospitalized for several days and eventually moved to our skilled care facility, Resurrection Life Center in Chicago. Father Matthias had also been diagnosed with prostate cancer several years before, but the urologist believed it was too slow growing to ever impact his life. He was at Resurrection Life Center for a couple weeks during which his health declined. He was put on hospice services. He died peacefully midmorning on Thursday, 2 May 2019.

Services were held at Saint Pius V Church in Chicago, Illinois on Monday, 6 May 2019. Visitation was from 3:00-7:00PM followed by Mass of Christian Burial. Fr. Charles Dahm, O.P., presided and preached. Funeral arrangements were handled by Smith-Corcoran Funeral Home. His body was cremated, and the remains were buried in the Dominican Section at Resurrection Cemetery in Madison, Wisconsin.