Only days after celebrating his 83rd birthday and the 54th anniversary of his ordination to the priesthood, Msgr. Tom Cassidy, in residence at St. Mark Parish in Vienna, died Oct. 1 during a medical procedure.

According to Jean Lupinacci, parish secretary, Msgr. Cassidy was undergoing a balloon angioplasty to prepare his heart for an experimental valve surgery he had scheduled later this month.

“He had a cardiac arrest during the procedure,” she said.

Msgr. Cassidy’s health had been declining for the last couple of months, said Claire Fountaine, who worked with Msgr. Cassidy for more than 20 years at St. Mark. But the priest had remained “very active” at the parish until as recently as two weeks ago, she said.

“The whole community of St. Mark is really going to miss him,” Fountaine said.

In a statement, Arlington Bishop Paul S. Loverde said that Msgr. Cassidy was present at the founding of the Arlington Diocese.

“He gave a shepherd’s care to the people entrusted to his pastoral care,” the bishop said. “His ministry embraced so many people. May the Good Shepherd give him eternal life and rest from his pastoral labors on earth.”

Msgr. Cassidy was born Sept. 25, 1928, to Patrick and Anna Laura Cassidy in Philadelphia. Growing up with simple roots gave Msgr. Cassidy the foundation for a life of humility, said Father Gerry Creedon, pastor of Holy Family Parish in Dale City, who served with Msgr. Cassidy at the diocesan mission in Bánica, Dominican Republic.

“He wasn’t someone who stood on ceremony,” Father Creedon said, adding that his background grounded him and gave him empathy for workers, especially those doing manual labor. Instead, Msgr. Cassidy was practical, straightforward and always concerned for others.

“He had the Church’s social mission and its sense for justice in his bones, in his blood,” Father Creedon said.

Msgr. Cassidy graduated from Holy Name College Seminary in Washington, D.C., in 1956, and was ordained to the priesthood by Richmond Auxiliary Bishop Joseph J. Hodges on Sept. 26, 1957.

A three-year stint as parochial vicar of St. Elizabeth Parish in Richmond, 1957-60, paved the way for 12 years of work with Catholic Charities and Catholic Family and Children’s Services. During that time, Msgr. Cassidy first met Pat Mudd, currently diocesan victim assistance coordinator, who was then spending her summers working for Catholic Charities. Working in an older building on Lincoln Street in Arlington had Msgr. Cassidy constantly on the lookout for repairs, Mudd said — and, in his straightforward way, trying to tackle them himself.

“He was always trying to fix it up,” she said. “And he had us doing it along with him.”

His time with Catholic Charities instilled in Msgr. Cassidy a love for children, the unborn and for adoption that “stayed with him all his life,” Father Creedon said.

In 1972, Msgr. Cassidy served as pastor of Sacred Heart Parish in Norfolk for one year before being appointed pastor of St. Mark.

His time at the new parish began tenuously, Fountaine said, as transition between leaders was difficult for the congregation. Msgr. Cassidy waited patiently to earn the parishioners’ trust, she said, and eventually was rewarded with their love. He also was responsible for the construction of the current St. Mark Church, an endeavor he put his “heart and soul” into, Fountaine said.

When Pope John Paul II elevated Msgr. Cassidy to the rank of monsignor on Dec. 18, 1982, the St. Mark staff had a party for him, Fountaine said. Surprised by their efforts, the newly named Msgr. Cassidy made it clear he was still, and always would be, simply “Father Tom.”

Msgr. Cassidy will be remembered for this humility, Fountaine said — as well as for his commitment to the Church.

“He was a great spiritual leader,” she said. “He enriched my faith.”

In 1989, Msgr. Cassidy was transferred to St. Anthony of Padua Parish in Falls Church, where he spent five years before moving to Bánica in 1994, joining Father Creedon.

“What struck me about him was his ability to be very practical about his concern for others,” Father Creedon said. “He had a very strong commitment to human dignity and to justice and to the good news … expressed in very practical ways.”

In Bánica, Msgr. Cassidy diverted rainfall runoff by shoveling shallow ditches, and he created more practical latrines, Father Creedon said. When a man needed a prosthetic leg, Msgr. Cassidy found one. When a woman needed dental work, Msgr. Cassidy subsidized it.

After leaving Bánica, Msgr. Cassidy served as pastor of St. Francis de Sales Parish in Purcellville, 1997-2003, before retiring at St. Mark. During his retirement, Msgr. Cassidy enjoyed celebrating Mass for students on Friday mornings, Fountaine said. He also enjoyed traveling.

Even in his last days, Msgr. Cassidy remained focused on the welfare of others. He needed the experimental cardiac surgery, but he didn’t want the doctors to move it up at the expense of the study.

“He was very concerned that this protocol not be changed in any way, so that it would help (not only) him, but many, many other people, too,” Mudd said.

Msgr. Cassidy didn’t live long enough to have the surgery, but he always knew he was being watched over, she said. “He had a great belief in God and believed He was going to take care of him.”

Before going in for the procedure that he never awoke from, Msgr. Cassidy was at peace, telling Mudd: “It’s all in the hands of the Lord now.”

Msgr. Cassidy is survived by a sister, Sarah A. Dandrea, her husband, Woody, and several nieces and nephews. A wake will be held at St. Mark Church Oct. 6 from 4 to 6 p.m. A funeral Mass will be offered at 7:30 p.m., with a reception to follow at the Parish Activities Center. Private interment will follow at a later date.