Toward the beginning of the film, Sharon is undercover as "Kate", a neighbor of Captain America. Sharon is a young, attractive S.H.I.E.L.D. agent with skills in martial arts and weapons whose mission is to guard Steve Rogers — and not let him know it. She is guided by a strong moral compass and is not afraid to speak out when her ethics are challenged.
Sharon Carter was assigned by S.H.I.E.L.D. in Washington, D.C. to monitor Steve Rogers, when Rogers arrives at his home apartment and was given the apartment opposite to his, acting as a nurse going by the name Kate. One day, Sharon Carter met Steve Rogers outside their respective apartments; after a brief chat, Rogers shyly asked Carter if she wished to join him for coffee, which she politely refused, noting she had to do her laundry. Later when Fury is attacked by the Winter Soldier she revealed her identity to Captain America.
The next day, Carter was at a meeting with Alexander Pierce, and at the end of that meeting, she came across with Steve Rogers, who ironically called "neighbor". Later, she questioned the orders from Jasper Sitwell because she did not understand why S.H.I.E.L.D. decided to hunt Captain America.
Agent Carter was present when Steve Rogers revealed that HYDRA was still alive. Also when Brock Rumlow and the STRIKE team walked to the control center to put the Helicarriers in orbit, Carter tried to stop Rumlow but he was more intelligent and manages to put the Helicarriers in orbit. After S.H.I.E.L.D. disbanded, Sharon was recruited by the CIA.
Two years after the fall of S.H.I.E.L.D., Shanon works for the CIA, being stationed at Bucharest following the fall of SHIELD.
When her aunt passed away, Carter attended the funeral in London. Carter gave a eulogy for her aunt; while giving the eulogy, Sam Wilson who attended the funeral, noticed Carter and told Steve Rogers, who was also in attendance and realized that Carter was related to his former love. In Carter's eulogy, she explained that she could never live up to her aunt so that was why she never revealed to anyone that they were related to each other.
At the wake, Carter and Rogers talked to each other with Rogers having only just learnt that she was Peggy Carter's niece. Rogers asked if she told Peggy about her spying on Rogers when they were neighbours and Carter told him that she didn't want to keep any secrets from her aunt about him. They then talked about the Sokovia Accords, where Rogers considered retiring from being Captain America, but Carter believed that Rogers could never stop as it suited him.
The two were interrupted by Sam Wilson who informed Rogers that the International Centre in Vienna had been destroyed. Carter, Rogers and Wilson all watched the news report saying that the bomb was set by James Barnes. Carter told Rogers and Wilson that she had to get back to Berlinand left the two of them behind. When Rogers and Wilson planned to go to find Barnes, Carter met up with them at a cafè and gave them some information about Barnes, including that her CIA team had orders to kill him.
In the comics, Sharon Carter is the niece of Peggy Carter. The original comic's time jump from the 1940s was only twenty years, making it plausible that Sharon was Peggy's niece. Despite the change in timeframe, she is still Peggy's niece in the films.
In the comics, Sharon eventually fell in love with Steve Rogers becoming the primary love interest.