McMullen was ordained to the priesthood in Rome on June 20, 1858, for the Diocese of Chicago. Archbishop Antonio Ligi-Bussi, O.F.M. Conv., was the ordaining prelate. He served the diocese, and later archdiocese, for 25 years. He was initially assigned as an assistant at St. Mary's Cathedral. During this time he helped to establish the House of the Good Shepherd, which cared for women who had been prostitutes, as well as orphanages for both boys and girls. He would go door to door to beg for money to support the institutions. He was well known at the Cook County jail and Bridewell, a house of corrections. In addition to visiting the inmates, he would bring newspapers, periodicals and books. He was known for his friendly demeanor and concern. From 1861 to 1866 McMullen served as president of the University of St. Mary of the Lake, during which time new facilities were built for the school. In 1865 he established the short-lived Catholic Monthly magazine while at the school. Unfortunately, the school suffered from a lack of finances and all but the seminary department was closed in 1866. He then founded St. Louis and St. Paul's parishes in Chicago. McMullen accompanied Bishop James Duggan as one of his theologians to the Second Council of Baltimore. In 1868 he was sent to the Holy See to represent the interests of the priests of Chicago after Bishop Duggan's mental illness became evident. He was named the pastor of St. Rose of Lima parish in Wilmington, Illinois, and started a new parish in Braidwood. He was named pastor of the Church of the Holy Name in Chicago and had just spent $19,000 on renovations when the Great Chicago Fire struck the city on October 8, 1871. Every structure McMullen had built in the city was destroyed. Once Father McMullen looked after his parishioners, he and other priests of the diocese traveled across the country and into Canada to raise funds to rebuild Chicago's churches, and to help the multitudes who were left homeless. He then built the present Holy Name Cathedral, which was consecrated on November 21, 1875. In 1877 he was named vicar general by Bishop Thomas Foley. After Bishop Foley's death he was named administrator of the diocese, and was renamed vicar general after the arrival of Archbishop Patrick Feehan.
Bishop of Davenport
On June 14, 1881, McMullen was appointed the first bishop of the newly created Diocese of Davenport. He was consecrated bishop by Archbishop Feehan on July 25, 1881, in Holy Name Cathedral. The principal co-consecrators were Bishops John Hennessy of Dubuque and John Lancaster Spalding of Peoria. Bishop McMullen chose St. Margaret's Church in Davenport to be his new cathedral and with zeal set to work to establish the new diocese. He was known for his simplicity of life. He did not own his own episcopal insignia or robes, but used those of the late Bishop Foley. Soon after arriving in Davenport he set out on a visitation of his diocese. He traveled by stagecoach, buggy, lumber wagon, hand car and passenger coach on the train. While on visitation he administered the sacrament of Confirmation. By December 1881 he confirmed over 7,000 people, and by the end of 1882, the number rose to 13,000. McMullen called the diocese's first synod in 1882 to set procedures and regulations for the new diocese. In September of the same year, he founded St. Ambrose, a seminary and school of commerce, for young men. Bishop McMullen's health soon failed, however. To try to find relief from his sufferings he attempted a trip to Rome, but only made it as far as New York. He also traveled to California where he fell gravely ill. A couple of months after his return to Davenport he died from stomach cancer after serving the diocese for a little less than two years. Archbishop Feehan celebrated the Requiem Mass and Bishop Spalding preached the sermon. He was initially buried in the crypt of St. Margaret's Cathedral, and then his body was transferred to the crypt of Sacred Heart Cathedral after it was built. Finally, he and the other bishops who had been buried in the crypt were transferred to the Bishop's Circle of Mt. Calvary Cemetery in Davenport.
Legacy
Both Holy Name Cathedral in Chicago and Saint Ambrose University in Davenport stand as monuments to his zeal. McMullen Hall, a classroom building at St. Ambrose, was named in his honor.
