Kristin grew up in Dorset, England, as the eldest of five children. Her father, a Royal Navy pilot, was killed in a flying accident when she was very young. Her mother then married another pilot, but Kristin's step-father then died in the same way as her father.

Kristin spent a year at London's Central School of Speech and Drama, and later said "It was disastrous..." Next, she began to train as a teacher, but her life changed for ever when she went on a visit to a friend in Paris for two weeks and found work there as an au pair, with a French couple who encouraged her acting ambitions. Still only nineteen, Kristin got a place at the École Nationale des Arts et Technique de Théâtre, in Paris, where she learnt the acting trade in the French language. A teacher at the school got her her first acting job, and she commented later "It helped that I was something exotic in France - a funny little English girl".

Kristin is married to a French doctor, François Oliviennes. They have a daughter, Hannah, and two sons, Joseph and George, and live in the country in northern France, with an apartment in Paris.

One of her sisters, Serena Scott Thomas, is also an actress.