The Catholic high school literally became home to D'Heygers, who initially slept on a bed in an unused restroom before moving into a converted nurse's infirmary. He later moved into a mobile home near the faculty cafeteria that eventually was replaced by a more modern one.
By the time an apartment was built for him above the new Campus Ministry Center in 2000, the unassuming D'Heygers had long become as much a fixture at Mater Dei as the statue of Mary in the grotto in the center of campus.
D'Heygers, whose dedication to the school was legendary, never married and never had children. But he established close bonds with staff, faculty and students.
"Mater Dei is my family," he said in his still-thick accent in an interview with The Times in 2000. "They are so good a family, so good to me. With them, I belong."
The man who has been called "the soul of Mater Dei High School" died Saturday of natural causes in his apartment on campus, said Patrick Murphy, the school's president. He was 94.
"The great thing is right to the end, he kept his keen mind and knew who everybody was," said Murphy. "He asked about how the basketball team had done, and so he was very aware of what was going on in the school."
At Mater Dei, Murphy said, "we looked on him two different ways. One was our caretaker and the other was our shepherd. He loved the students and the faculty and staff. "He was a very gentle, very kind man, an extremely hard worker and extremely dedicated."
D'Heygers was born in Belgium on April 17, 1915, with a deformed right ear that impaired his hearing. He told The Times in 2000 that his malformed ear made him an outcast as a child and that he had few friends.
After he dropped out of school in the eighth grade, his parents sent him to work in a shoe factory.
D'Heygers, who worked on forged documents for the Belgian underground during World War II, had been a shoemaker for more than two decades when an old friend from Belgium who was working as a janitor at Mater Dei wrote him about an opening for another janitor.
By then, D'Heygers' parents were dead and his sister was married and had a family of her own.
"Nothing stopped me," he said of his decision to move to America in a 2005 interview with The Times. "I was by myself."
D'Heygers' work day for many years began at 4:30 a.m. when he got up to unlock the main gate to let the bakery truck in. It ended at night when he checked the locks on the doors and made sure all the lights had been turned off.
In between, he'd drive the school's nuns to and from the convent, clean, replace light bulbs and do general maintenance.
"He had the keys to everything and was here all the time," said Murphy.
"Why should I leave?" D'Heygers said with a laugh in the 2005 Times interview. "I am very happy here."
For decades, D'Heygers attended daily Mass in the campus chapel. And he could often be found standing in front of the statue of Mary.
"I pray a lot, but I never pray for me," he said in the 2005 interview."I never ask for anything for me. I just say, 'Thank you.' "
In 2002, he expressed his thanks by willing his entire estate to Mater Dei.
The amount was not revealed. But it was said to be well into six figures, thanks to his penchant for saving and investment help from a school parent.
Although Murphy said D'Heygers technically retired in the late 1990s when his doctor said he could no longer get on ladders, he continued to make his daily rounds until about five years ago.
And he continued to be a presence on campus, where his apartment was filled with Mater Dei memorabilia. Also, the Academic Service Center clock tower that bears his name is visible from his living room window.
"When I am gone, my name will remain," he said in the 2005 interview. "I will always be a member of this family. I'm very lucky."
D'Heygers had no immediate surviving family members.
A Rosary for D'Heygers will be said at 6:30 p.m. Monday in the Meruelo Athletic Center at Mater Dei High School, 1202 W. Edinger Ave., Santa Ana. Immediately following the 7 p.m. funeral Mass, an on-campus reception will be held in the Levecke Center.