Sister Mary Mark Mahoney grew up on a farm in Minnesota, taught grade school for 35 years, and helped start a house of prayer in Stillwater. But it wasn’t until the Sister of St. Joseph of Carondelet retired at the age of 84 that she discovered her greatest passion.

When Sister Mary Mark spotted an ad in National Catholic Reporter asking for volunteers to correspond with death row inmates, she thought to herself, “I could do that.”

And so began a prison ministry that has lasted 16 years and counting.

Now, Sister Mary Mark, who will celebrate her 78th jubilee this year, is 100 years old. But age hasn’t stopped her from doing what she describes as the best thing she’s ever done.

Sister Mary Mark began corresponding with two inmates — one at San Quentin State Prison in California and another at Leavenworth Penitentiary in Kansas. Soon, however, she began writing to other prisoners, male and female. At one point, she was writing to 50 prisoners.

“Mostly what I do is encourage them to do the best they can where they are,” she said. “I respond to what they say and I tell them what’s going on with me. We really do have a wonderful relationship.”

When she was 89, Sister Mary Mark traveled to Oklahoma City to testify on behalf of a man with whom she had been corresponding for four years. Although she still believes he was innocent, the jury found him guilty and he was executed.

“He was one who studied the Bible,” Sister Mary Mark said. “I’d say he was a really good man.”

Sister Mary Mark said she realizes many people don’t understand how she can write to murderers and criminals. To that, she responds we are all children of God.

The ministry has made her a more compassionate person, she said. “It’s the best thing I ever did, and I will continue on until I can’t do it anymore.”

Other Sisters of St. Joseph of Carondelet celebrating 100th birthdays this year are: Sister Ann Lucille Wallenta, Sister Constance Marie DeFoe and Sister Mary Isidore Louwagie.