Mary Marg Helgenberger (born November 16, 1958) is an American actress. She began her career in the early 1980s and first came to attention for playing the role of Siobhan Ryan on the daytime soap opera Ryan's Hope from 1982 to 1986. She is best known for her role as Catherine Willows in the CBS police procedural drama CSI: Crime Scene Investigation (2000–13) and the subsequent TV movie Immortality (2015). She reprised the role in the sequel CSI: Vegas (2022–present). Helgenberger is also known for playing the role of K.C. Koloski in the ABC d…