Hayley Smith is the daughter of Stan Smith, a CIA agent, and Francine Smith, a housewife. Hayley was born on a safari in Africa, due to Stan refusing to believe the OB-GYN when Francine said she should not travel. Unlike her father, her mother and her brother, Steve Smith, Hayley is ultra-liberal. She is a casual user of marijuana (though in "Jones for a Smith," Hayley actually worried that her marijuana smoking is getting out of hand and begs Francine to let her go to rehab, with Francine callously telling her daughter to stop being so melodramatic). Hayley often helps the homeless, and is in favor of gun control. This causes a great amount of distrust for Stan, consistently having views being polar opposite of hers, but deep down they are more alike than they think. When she was younger, Hayley and Stan got along very well, and indeed, they have similar personality traits, such as stubbornness and distrust of those they disagree with. In the third season episode "Stanny Slickers II: The Legend of Ollie's Gold", it is revealed that her middle name is Dreamsmasher.
She lives with her parents and goes to Groff Community College, though she moved out temporarily after a bitter argument with Stan. As a college student, Hayley also majors in women's history and promotes women's rights (though she downplayed the blatant gender oppression of the Middle East, even telling Saudi guards that she respected their right to chase her). Hayley is very often the rational and open-minded member of the family. For example, she helped to unionize the homeless men Stan paid to fight each other, helped free foster children that Roger had enslaved, and is the one most likely to stand up for people's rights when her family tries to exploit people. Nevertheless, she is also sometimes seen as hypocritical and has moments of weakness—for example, in "Camp Refoogee" she went to an African refugee camp and swore to help the starving people during the short time she expected to be there. However, after finding out they would be there for a few weeks, she went to the spa-like U.N. aid base and went so far as to eat steak, despite being a vegetarian. Other members of the household seem to find her annoying, even Roger, especially after she verbally abused him for voting for President George W. Bush.
The fourth season has started showing that Hayley is often prone to violent and uncontrollable mood swings. During puberty, she went through violent outbursts during every development, such as having to wear tampons (and ruining the family's white couch by sitting on it while wearing a skirt), being disappointed over her small breasts, and getting her first pimple. These mood swings terrified her parents and they were fearful of the same problems in her brother, although it is shown Steve has had his temper more under control. Another episode revealed that Hayley will fly into a rage if a man breaks up with her. It has gotten to the point where Hayley will have to go to jail if another relationship she has falls apart.
It was revealed in "The Kidney Stays in the Picture" that her father may be a man named Joel Larson who Francine slept with three days before she and Stan got married. When Stan and Francine go back in time to 1996 to find out the man's identity, Stan almost jeopardizes her existence when he confronts the younger Francine, reminding her that she's getting married and shouldn't be sleeping with other men. Stan donated his kidney, due to Hayley needing a transplant, without knowing the results of the paternity test and said he never wanted to know whether he was Hayley's father. When the doctor asked how he got the kidney, Stan said, " I called in a favor." It was later revealed that Roger cut into Larson's chest, getting the kidney.