Derek was born Mary Cathleen Collins in Long Beach, California. She is of Irish, German, Dutch and Welsh descent. Her father is Paul Collins, a Hobie Cat executive. Her mother is Norma Collins, hairdresser to Ann-Margret. After the parents divorced, Norma Collins married American stunt performer Bobby Bass.
Derek attended Narbonne High School in Harbor City, California. Derek then attended George S. Patton Continuation school which is adjacent to Narbonne.
Derek started modeling to pay for a new surf board, and after a few shoots and a deodorant advertisement found she had a knack for performance. She got herself a Hollywood agent, and on the audition trail met director John Derek, 30 years her senior. John filed for divorce from his wife, actress Linda Evans, and the pair moved to Germany to avoid John being charged with statutory rape under U.S. law due to Bo only being 16. The couple returned to America soon after Bo’s 18th birthday, and married in 1976. They remained married until his death in 1998.