Born in Los Angeles, California, Hamilton as a young woman initially wanted to be a model but could not find work in the fashion industry owing to her short stature and race. Instead, she enrolled in acting classes after seeing an advertisement in the Los Angeles Times and then enlisted the services of an agent. Hamilton made her professional acting debut in the 1950s television sitcom Amos 'n' Andy. She played the girlfriend of Andy (Spencer Williams) on the show for several episodes.

Hamilton appeared in more than 60 television series and television films throughout her career. In 1960, she guest-starred in an episode of The Twilight Zone called "The Big Tall Wish." In 1963 and 1964, she played a high-school librarian on two episodes of the popular series My Three Sons. She also became one of the first black actresses to appear on the soap opera Days of Our Lives. Other roles included guest spots on The Thin Man, General Hospital, Sanford and Son, Good Times, The Jeffersons, In the Heat of the Night, All In The Family, and Law & Order. She portrayed Songi in "Final Mission" a 1990 episode of Star Trek: The Next Generation. Her last television credit was a 2008 episode of the ABC series Private Practice.

Hamilton portrayed, in an uncredited role, Helen Robinson in the 1962 film adaptation of To Kill a Mockingbird, based on Harper Lee's novel of the same name. She was the film's last surviving African-American adult cast member with a speaking role.

Kim Hamilton was married three times. Her first marriage, at age 18, was to Robert Henry Hamilton in 1951. They had two children but divorced a decade later. One of her children, her son Robert, predeceased her. Her second marriage was to television Director Dave Geisel, which lasted only three years, from 1962 to 1965. She then dated German-born actor Werner Klemperer for more than two decades before they married in 1997. They remained together until Klemperer's death on December 6, 2000.

In December 2007, Hamilton was honored for her career achievements by Columbia University and the Harlem community at an event held at the Museum of the City of New York. Hamilton's honor was part of series of Columbia University's Big Read program, focusing on To Kill a Mockingbird through guest lectures, productions, and panel discussions.

Hamilton in her later years divided her time between the East and West coasts, living at her home in Manhattan and at her other residence in her hometown of Los Angeles. In 2013, at age 81, she died of undisclosed causes while in Los Angeles.