Lacking funds and concerned about his family, McGivney went home for his father’s funeral and lingered awhile in Waterbury. Then, at the request of the bishop of Hartford, he entered St. Mary's Seminary in Baltimore, Md. After four years of study, on December 22, 1877, he was ordained in Baltimore's historic Cathedral of the Assumption by Archbishop (later Cardinal) James Gibbons. A few days later, with his widowed mother present, he offered his first Mass at Immaculate Conception Church in Waterbury.
Father McGivney began his priestly ministry on Christmas Day in 1877 as curate of St. Mary's Church in New Haven, the city's first parish. The original Church of St. Mary’s was destroyed by fire and so a new stone church had been built on Hillhouse Avenue, one of New Haven’s finest residential streets. However, since Catholics were not liked, to say the least, there was neighborhood objection to the Catholic Church which even the New York Times noted in 1879, under the headline: “How An Aristocratic Avenue Was Blemished By A Roman Church Edifice.” So, Father McGivney’s priestly ministry in New Haven began with tension and defensiveness among the working-class Irish families he served.
One of the responsibilities of St. Mary's priests was pastoral care of inmates in the city jail. In a notable case, a 21-year-old Irishman, while drunk, shot and killed a police officer. James (Chip) Smith was tried for first-degree murder in 1881, convicted and sentenced to be hanged. Father McGivney visited him daily.
After a special Mass on the day of execution, the priest's grief was intense. The young offender comforted him: "Father, your saintly ministrations have enabled me to meet death without a tremor. Do not fear for me, I must not break down now."
Father McGivney worked closely with the young people of St. Mary's parish, holding catechism classes and organizing a total abstinence society to fight alcoholism. In 1881 he began to explore, with various laymen, the idea of a Catholic fraternal benefit society. In an era when parish clubs and fraternal societies had wide popular appeal, the young priest felt there should be some way to strengthen religious faith and at the same time provide for the financial needs of families overwhelmed by illness or death of the breadwinner.
He discussed this concept with Bishop Lawrence McMahon of Hartford, and received his approval. He traveled to Boston to talk with the Massachusetts Catholic Order of Foresters, and traveled to Brooklyn to consult the Catholic Benevolent Legion. He met with other priests of the diocese. Wherever he could, he sought information that would help the Catholic laymen to organize themselves into a benefit society.