On 28 January 1663, Françoise married Louis Henri de Pardaillan de Gondrin, Marquis of Montespan, who was one year her junior.
Madame de La Fayette says in her Histoire de madame Henriette d'Angleterre that Françoise-Athénaïs was in love with another young man, Louis de La Tremouille, who was the elder son and heir to the Duc de Noirmoutier (one of the leaders of the Fronde). However, La Tremouille had to flee to Spain after a disastrous duel, and Françoise-Athénaïs was betrothed to Montespan. The wedding ceremony took place in a chapel at the Église Saint-Eustache in Paris.
She soon became pregnant with her first child, Christine (b. 17 November 1663). Two weeks after her daughter's birth she danced in a Court Ballet, and less than a year later her second child, Louis Antoine was born (b. 5 September 1664).
The couple lived in a small house close to the Louvre, which allowed Madame de Montespan to attend court and carry out her duties there as a lady-in-waiting to the Duchess of Orléans. She quickly established herself as the "reigning beauty of the court". Beauty, however, was only one of Madame de Montespan's many charms. She was a cultured and amusing conversationalist, who won the admiration of such literary figures as letter-writer Madame de Sévigné and diarist Saint-Simon. In addition, she kept abreast of political events. This had the effect of making her even more appealing to men of intellect and power. She was courted by a number of suitors including le comte de Frontenac and Marquis de La Fare.
However, for Mademoiselle de Mortemart, an alliance with a quite obscure noble family from south-western France was not enough and her husband (always short of money) was always away on his judicial duties.
By 1666, Madame de Montespan was trying to take the place of Louis XIV's current mistress, Louise de La Vallière. Using her wit and charm, she sought to ingratiate herself with the king. She became the mistress of Louis XIV in 1667. When her husband found out, instead of accepting it as was usual to cuckolded husbands of the era (especially when it was the king doing the cuckolding), he raised a scandal at court, challenged the king one day at Saint-Germain-en-Laye and decorated his carriage with antlers (like horns, a traditional symbol of the cuckolded husband). He was promptly imprisoned in the For-l'Évêque, then exiled to his lands. This did not stop his agitation, and not only did Montespan go to Paris nearly every year between 1670 and 1686, he also commanded an annual requiem mass to be sung for his wife during her lifetime.