And the miracles always happen.
When she needed money to pay taxes on the church property, St. Stephen's, it was donated by good Samaritans in the exact amount. Same for the utility bills.
St. Stephen's Church works in conjunction with the nonprofit Samaritan Social Services in Muscoy to serve the poor - and their traditional pre-Thanksgiving Day dinner, now in its 28th year - is part of it.
On Monday, McKinney, 73, began preparation - roasting 55 turkeys, making rice dressing and 275 pounds of potato salad, cooking 300 pounds of yams and 200 bunches of collard greens for a Southern-style Thanksgiving dinner in the front yard of St. Stephen's Church.
Volunteer Rosa Camberas of Mentone chopped boxes of fresh yams.
"It makes me feel good to help," she said.
Then there was dessert - 50 pumpkin pies, 30 apple pies and peach cobblers, and 10 huge sheet cakes.
Throughout the day Wednesday, McKinney and the 100 volunteers served dinner to more than 800 people.
As people left, there was a giveaway of 350 Thanskgiving food bags, complete with chicken.
Director of volunteers Von Sommerville of Rancho Cucamonga has been working with McKinney for 17 years.
"Mary is an Energizer Bunny, almost 25 years my senior, and runs circles around me," Sommerville said.
"She treats these people with care, dignity and respect and tells the volunteers to treat everyone like they were president of the United States."
One miracle this year was receiving fresh produce through Bishop Albert Scales, founder and CEO of Victory City, a community garden project, and Linda Wright, president of Wright on Time Youth and Family Services.
"Also, the Food Bank in San Bernardino has just been a Godsend to the people of this area," McKinney said.
"They share everything they get. Without the food bank, we soup kitchens wouldn't be able to do it."
Gerald Wilson, program manager at the nonprofit Community Action Partnership of San Bernardino County Food Bank, said the food bank has about 200 affiliated nonprofits in the county and acts as a hub for smaller churches.
"We fed nearly 32,000 this month through the agencies served by the Food Bank in San Bernardino," Wilson said.
The Food Bank has worked with McKinney for years and over time, she has won several service awards, according to Wilson.
"Sister Mary donates to people in need who can't help themselves."
From noon to 4 p.m. on Wednesday, there were also clothing giveaways, music, arts, crafts and activities.
Holidays aren't the only time McKinney and Co. fill hungry bellies.
Each Thursday throughout the year, McKinney manages to feed a hot meal to more than 140 people.
Those she ministers to - the working poor and the homeless - stand in line throughout the year to get the donated bags of bread, cereal, canned vegetables and soups.
Sister McKinney and her husband, the former Rev. Jesse McKinney, travel throughout San Bernardino, feeding and ministering to the city's poorest residents.
The couple have been married 35 years.
"He's another miracle in my life," she said.