On February 3, 1933, Ritter was appointed as auxiliary bishop of what was then the Diocese of Indianapolis and titular bishop of Hippos by Pius XI. He received his episcopal consecration on March 28, 1933, from Bishop Chartrand, with Bishops Alphonse Smith and Emmanuel Ledvina serving as co-consecrators. As an auxiliary bishop, he also served as vicar general of the diocese from 1933 to 1934. At age 40, Ritter was one of the youngest Catholic bishops in the United States. Following the death of Bishop Chartrand, Ritter was appointed the seventh bishop of the Diocese of Indianapolis on March 24, 1934. In 1938, he directed an end to racial segregation in all Catholic schools in the diocese. His decision, made 16 years before Brown v. Board of Education, was met with opposition by the Ku Klux Klan, whose members protested outside of SS. Peter and Paul Cathedral, and even by his fellow members of the clergy. Ritter also reorganized the diocesan Catholic Charities, introduced the Catholic Youth Organization, and completed the construction of SS. Peter and Paul Cathedral. He reduced the diocese's debt by $3 million, . Ritter also reorganized the Catholic Charities in the diocese. In 1941, Ritter opened St. John's Parish in Evansville, Indiana, the first Black Catholic parish in the city. The Diocese of Indianapolis was elevated to the status of an archdiocese by Pope Pius XII on October 21, 1944, and Ritter was installed as its first archbishop on December 19, 1944. St. Louis years Ritter was appointed the fourth archbishop of the Archdiocese of St. Louis by Pius XII on July 20, 1946, succeeding the late Cardinal John J. Glennon. St. Louis grew quickly during the post-World War II economic boom. Ritter opened an average of three parishes per year in St. Louis city and county. An able administrator, without the authority to tax or compel, Ritter nevertheless raised more than $125,000,000 () to build sixty new parishes, sixteen high schools, and the Cardinal Glennon Memorial Hospital for Children in St. Louis during his tenure there. Desegregation efforts As one of his first acts as archbishop, Ritter announced that Webster College would accept black students, endorsing the Sisters of Loretto's wish to enroll black women in their Webster College (which Ritter's predecessor, Archbishop Glennon, had ruled impossible).He also allowed the senior class of St. Joseph's High School, then the city's only black Catholic high school,