MacLachlan was born at Yakima Valley Memorial Hospital in Yakima, Washington. His mother, Catherine (née Stone; 1933-1986), was a public relations Director for a school district and a homemaker who was active in community arts programs. His father, Kent Alan MacLachlan (born McLachlan; 1933-2011), was a stockbroker and Lawyer. Kyle has Scottish, Cornish, and German ancestry. He grew up in Yakima, the eldest of three children. His two younger brothers, Craig and Kent Jr., live in the Seattle area. Kyle graduated from Eisenhower High School in Yakima. His mother left his father when he was 17. His parents divorced in his senior year of high school in 1977.