St. Genevieve is the patroness of Paris because when Attila was on the march, she calmed the panicking citizenry, exhorting them to pray and fast to avert disaster rather than evacuating. People calmed, prayed as she asked, and Attila abruptly changed course and the city was spared. When Childeric laid seige to Paris years later, Genevieve snuck through the lines on a boat and brought grain to starving residents. She also appealed personally to him for mercy for POWs, which she obtained. The prayers of one girl against Attila and Childreric!
She became an adviser to Clovis, persuading him to be more merciful to lawbreakers, and to build a church. During her life she was famed not only for these interactions, but also for her great kindness to the poor and her astonishing asceticism (she was said to eat only twice a week until her directors persuaded her otherwise).
Not that she didn't have her trials. Like Joan of Arc, she was frequently persecuted and mocked as an impostor or some kind of nut. There was even an effort to drown her (!) until St. Germain --who sort of "discovered" her at a young age-- intervened with a vote of confidence.
After her death there were many miracles associated with her, the most dramatic being the sudden cessation of a plague ("burning sickness") that killed 14,000 people after a procession in her honor. Thereafter, Genevieve's relics were carried in procession yearly until the revolution, when many of them were destroyed.
Here's a poem from WWI invoking Genevieve & Joan:
Litany
Allene Gregory
Allene Gregory
Saint Genevieve, whose sleepless watch
Saved threatened France of old.
Above the ship that carries him
Your sacred vigil hold.
Where all the fair green fields you loved
Are scarred with bursting shell,
Joan, the Maid who fought for France--
Oh, guard your young knight well.
But if by sea or if by land
God set death in his way--
Then, Mother of the Sacrificed,
Teach me what prayer to pray!
God set death in his way--
Then, Mother of the Sacrificed,
Teach me what prayer to pray!
