Murder, accidental deaths and betrayal were par for the course in her Paris trilogy, "Le Divorce," "Le Marriage" and "L'Affaire." Johnson used those books to weave pitch perfect observations of love, cultural differences and the lives of American expatriates.
Dark events weren't so much central themes as unpleasant discoveries that nonetheless tipped the tales in uncomfortable directions.
So is Johnson a comic writer with a taste for the minor key? A portrait artist who can't help including the out-of-place knife at the edge of the painting? Her new book, "Lulu in Marrakech," only deepens the mystery.
Lulu Sawyer is a CIA agent from California dispatched to Morocco for long-term intelligence work in the war on terror. She has what appears to be the perfect cover: an English boyfriend, Ian, with whom she plans to live and who shares an interest in continuing the type of social work that first drew them together in Kosovo.
Her job: Trace Moroccan links in the pipeline funneling donations to al-Qaida.
Lulu acknowledges up front a negative attitude toward her latest assignment.
"For one thing, I was a little frightened of Islam; after all that's happened, who isn't?" she says. "Maybe Muslims themselves are afraid of it, disconcerted to find themselves prisoners of a situation where even their families and people they know might turn on them and blow them up."
The book develops gradually, subtly, almost as a novel of manners, with understated scenes of dinner parties and the parsing of characters' love lives. Along the way, we're treated to artful descriptions that belie the trouble ahead.
Here's Lulu on the plane, heading into Marrakech: "We were flying a bit lower, so that now the cities of the northern coast were visible on the edge of the sea, arcs of settlement like white rickrack against the turquoise Mediterranean."
Yet almost immediately you sense a crackle of intensity below the surface of Lulu's quiet life. After all, it's clear that not everyone within Lulu's small circle of Moroccan acquaintances is who he or she seems to be.
Is one of them an al-Qaida operative behind the funding Lulu is trying to investigate? Surely not Robin Crumley, the tweedy English poet whose younger wife, Posy, is expecting a baby. What about Gazi and Khaled, the well-appointed Saudi couple who socialize with Ian?
Surely it couldn't be Ian himself, whose work in Morocco is not entirely transparent and who disappears for unexplained stretches of time. And what about those glimpses Lulu has of Ian and Gazi together? Does Lulu have a rival for Ian's love?
As Lulu sorts out these mysteries, a concrete assignment arrives. She is to help with the abduction of an Algerian Muslim living in Paris who has come to see his sister in Morocco.
The girl was taken to the country to protect her from an honor killing involving this brother and other members of her family.
It's during this abduction that Johnson provides the lurch that, in hindsight, we expected all along. At first, it's enough to cause you to rethink everything you thought you knew about what was happening.
But then you remember this is Lulu we're talking about, a spy fully dedicated to her profession. While given to very real emotions, she also seems cold-bloodedly suited to her life's calling.
Her ruminations on the likely fate of the abductee, after all, are hardly those of a bleeding heart. And it is within this paradox, of regretting a painful action even while accepting its consequences, that the heart of the novel lies.
"You have to be willing to be a person who changes things, even if you risk changing them for the worse," Lulu observes.
"It seemed hard, though, to imagine consigning to torture someone whose sister you knew," she continues. "I recognized this was a very Californian qualm, to hope that the evil you do won't count, like a video game."