They weren't in the market for a vacation place. John, who had been in the military, was building a real estate career; Marilyn was building an antiques business.
"I didn't know how I'd have time for a second home," John says.
Let alone two second homes.
The main house, with its astonishing center hall, had been built in 1909 by Baltimore's Rice family, which ran a bakery business from the 1870s until the early 1970s.
Steps away, they built a second, smaller building, where the servants lived and did the cooking so the main house and its sleeping rooms would stay cool.
"It just seemed like we claimed it the minute we walked in," Marilyn says.
These second homes were bought not in near-retirement with the accumulated wealth of a long and successful career, but in the early days of married life, when money was tight and the demands of work left little time to relax.
Nor is this the story of a waterfront show house, although Marilyn's decorating skills have drawn the attention of two magazines. Instead, it is the story of a pair of houses refined and restored during what amounted to a quarter-century of weekend projects.
"Every weekend, we did something," John says. "We didn't have a vision of what it could be. We just knew we couldn't come here and not do something."
All these years later, there are still plenty of projects - Marilyn paid a neighbor child to help her cut out the wallpaper dogs (10 cents per dog) that she used to decorate the wainscoting in a powder room. John is working against nature to reinforce the shoreline with huge rocks.
But the work stops when friends arrive.
"It looks great today, but this has been 25 years," John says. "We liked the idea that it was on the water, but it has added something else to our lives."
The center of the couple's life here on the outskirts of St. Michaels is the L-shaped screened porch that wraps around the corner of the main house. That's where they entertain a sprawling extended family of friends and Bethesda neighbors who visit just about every weekend from April to October, along with an assortment of children and dogs.
There might be six or 16 around the 7-foot Plexiglas tabletop that is anchored to a trio of iron gates - among many antiques Marilyn found and adapted to the house.
The only electricity on the porch runs the ceiling fans, which, along with the evening breezes from the bay, cool the porch and tease the candles and oil lamps that light the comfortable companionship of the diners.
When evening is done, the guests retire to the three bedrooms on the first floor of the main house, and the Hannigans return to the old cooking house, which has been converted to a comfortable cottage for them.
When you spend a weekend with friends at such a place, John says, you get to know them. The pair of houses have any number of spots, indoors and out, to which hosts and guests can retreat for comfort and conversation.
"I was out with the dogs one morning, and I found a perfect spot," he says, referring to a corner of the cottage's tiny porch. "I've been here all these years, but I thought, 'This is perfect. Right here.' "
The main house was a gloomy fishing retreat, paneled in dark wood, when the Hannigans bought it in 1982. They sold most of the dark wood furniture they found inside and used the $1,500 to buy the bamboo furniture that still fills the living room.
Marilyn painted everything white - floors, furniture, walls - and chose blue fabrics in stripes, plaids and florals for every room. "The architecture and the setting dictates, and blue and white emphasize, the water," she says.
Both houses are decorated with folk art, botanical prints, furniture and antique linens Marilyn found while traveling for business.
In the main house, guests can find straw hats, well-worn paperbacks, and stacks of magazines, board games, and a wide-screen TV. A telescope is perched by a dormer window on the second floor.
The wind off the water makes living here almost unthinkable in the winter, despite the winterizing that was one of the Hannigans' many projects.
Come October, they will close up the houses and pick up their city life in Bethesda, just as they have for the last quarter-century.