Carrie is introduced in Season 2, where she worked at the CIA headquarters in Los Angeles with Marshall. Eventually the two formed a relationship resulting in marriage and the birth of their son, Mitchell. This, of course, happened in typical Marshall fashion. The two decided they were to be wed before the birth of the baby, but Carrie began to go into labor in the middle of an important mission. Marshall quickly enlists Eric Weiss to become legally ordained online (through the "Internet Church of Mammals") so he can marry the two (while Marshall continues to guide Sydney and Vaughn through their mission), bolting at the last second to take his wife to the hospital.
Carrie is strong-willed and tends to get her way in disputes with her husband, and though she tries to keep him in check with her quick wit and sarcasm, they ultimately balance each other out and love one another deeply.
Beginning in the fourth season, Carrie seems to have given up her NSA job to take care of Mitchell. She is unaware Marshall still works for the CIA, since APO is black ops and he is under orders to keep it secret. She only appeared in one fourth season episode ("Tuesday"), and prior to the series finale appeared in one episode during the fifth season (...1...).
For the series finale, Carrie learned Marshall was still with the CIA when he was kidnapped by agents of Arvin Sloane. She worked with APO to rescue Marshall and the also-abducted Rachel, demonstrating technical prowess rivalling Marshall's.
In the flash forward several years into the future presented at the end of the series finale, it is mentioned Marshall and Carrie have two more children after Mitchell, with another on the way, and all are boys.