Born in Appleton, Wisconsin, on August 7, 1901, Walter Henry Van Rooy was one of the ten children of Henry and Johanna Van Rooy, and the twin brother of Ruth M. Van Rooy. He attended St. Mary's Parochial School and Appleton High School in Appleton. Following three years of college work at St. Francis Seminary in Milwaukee, Wisconsin, he entered the Dominican Order at St. Joseph's Priory, Somerset, Ohio, receiving the habit and the religious name of Daniel on September 25, 1924. A year and a day later, he professed simple vows at St. Rose Priory in Springfield, Kentucky.

Philosophical studies were taken at the Dominican House of Studies in River Forest, Illinois, and theological courses at the House of Theology in Washington, D.C. His ordination to the priesthood took place on June 15, 1931, at St. Dominic's Church in Washington.

In 1932, he was awarded the Master of Arts degree in English from the Catholic University of America and was then assigned to Fenwick High School where he taught English and religion from 1932 to 1934. For the next two years, he worked as an associate pastor at Blessed Sacrament Priory in Madison, Wisconsin, and studied German literature at the University of Wisconsin. He returned to Fenwick in 1936 and taught Religion, English, Latin, and German there until 1960; during that time, he also served as principal of the high school from 1943 to 1946, and as building fund director from 1946 to 1948.

In 1960, Father Van Rooy was appointed pastor of St. Albert the Great Parish in Minneapolis as well as superior of the Dominican community and served there for three years. From 1964 to 1967, he was Chairman of the Theology Department and taught philosophy and theology at Presentation College in Aberdeen, South Dakota; he also fulfilled the duties of chaplain at the College and at the Motherhouse of the Presentation Sisters. From 1967 to 1976, his teaching career continued on the faculty of Bishop Lynch High School in Dallas, Texas. When ill health forced him into limited service, he remained assigned to Saint Joseph Priory in Dallas until his return to Chicago in 1981, the year of his fiftieth anniversary of ordination to the priesthood. Father Van Rooy spent the last four years of his life in Appleton, Wisconsin, caring for his aging sisters. On August 10, 1985, he suffered a stroke and thereafter began to fail rapidly. He died a peaceful death on November 10, 1985. Following a funeral Mass at thePrioryof St. Dominic and St. Thomas in River Forest, he was buried in the Dominican plot at All Saints Cemetery in Des Plaines, Illinois, on November 13.