In Fahrenheit 451, Mildred is never directly asked if she is happy but, at the end of Part One, she declares that she is happy to Montag as he bemoans his unhappiness:
"I am." Mildred's mouth beamed. "And proud of it."
To put this into context, she has just advised Montag to "take the beetle" and go driving, as a means of making himself happy. According to Mildred, it feels "wonderful" to go out into the countryside and drive fast, sometimes for the entire night. Evidently, this is how she copes with bouts of unhappiness when they occur.
It is ironic that Mildred would declare her happiness in the novel, considering that she attempted suicide just a little while earlier. The fact that she will not admit it, even to her husband, demonstrates the extent of her self-imposed repression and her need to appear happy on the outside, no matter how she feels inside.