Born in Vail, Colorado,^[2] Shiffrin is the second child of Eileen (née Condron) and Jeff Shiffrin, both originally from the Northeastern United States and former ski racers;^[12][13] her mother became a nationally prominent masters racer.^[14] Her paternal grandfather was Jewish.^[15] Shiffrin's father Jeff grew up in New Jersey and was an avid skier on weekends in Vermont with his family. As an undergraduate, he raced for Dartmouth College in New Hampshire.^[16] Her mother Eileen raced in high school in northwestern Massachusetts in the Berkshires,^[13] and her brother Taylor (born 1992) raced for the University of Denver.^[17]

When Mikaela was eight in 2003, the family moved to rural New Hampshire near Lyme,^[18] where her father, an anesthesiologist, worked at Dartmouth–Hitchcock Medical Center. After five years, he took a new job in Denver;^[19] Shiffrin's older brother Taylor was in high school at Burke Mountain Academy, a ski academy in northeastern Vermont, and stayed in the east. Shiffrin also attended middle school at Burke but went with her parents to Colorado before returning to Burke.^[16][20]

From a young age, Mikaela had strong results in major competitions. In March 2010, at age 14, she won both the slalom and giant slalom (GS) at the Topolino Games in Italy, against skiers from 40 nations.^[21] The following winter, now meeting the FIS minimum age requirement of 15 years, she won a Nor-Am Cup super combined race in December 2010 at Panorama, British Columbia, only the eighth FIS-level race in which she had competed. Shiffrin followed it up with three podiums in her next three Nor-Am races: runner-up in a super-G, third in a GS, and victory in a slalom. Weeks later, she won a pair of Nor-Am slalom races held at Sunday River, Maine. A month later, Shiffrin took the slalom bronze medal at the FIS Junior World Ski Championships held at Crans-Montana, Switzerland (after having been down with a stomach virus the day before).^[22]