Michael Castleman was born on February 2, 1950 in Brookline, Massachusetts to parents of Ukrainian descent. He was named after a deceased relative, per Jewish tradition. He grew up in Lynbrook, a Long Island suburb of New York City. His father was a professor of metallurgy at Polytechnic Institute of New York University in Brooklyn. His mother was a school librarian. Castleman graduated from Malverne High School in 1968 and attended the University of Michigan, graduating Phi Beta Kappa in 1972. He earned a Masters in Journalism from the University of California, Berkeley, in 1977.
In college, Castleman spent most of his non-academic time working to end the war in Vietnam. After graduating, his anti-war efforts led to a job doing outreach for the Ann Arbor Free Clinic. Castleman quickly saw that most of the patients had problems that could have been easily prevented with good health information. He approached the underground newspaper, the Ann Arbor Sun, with a proposal to write regular health articles. For three years (1973-1975), he wrote about optimal wellness, staying healthy, mainstream medicine, alternative therapies, nutrition, fitness, and sexuality. He has written about those subjects ever since and has been called "one of the nation's leading health writers" (Library Journal).
Arriving in San Francisco in 1975, he wrote health articles for Pacific News Service, a small anti-war press syndicate that, after the Vietnam war, published news the mainstream media ignored. To supplement his writing income, Castleman became the director of the San Francisco Men’s Reproductive Health Clinic, the nation’s first birth control clinic for men. It distributed thousands of condoms for free before the AIDS epidemic when the devices were not sold openly. To bring men in, clinic staff were trained by the University of California, San Francisco Medical Center’s Human Sexuality Program in brief-intervention sex counseling. Castleman and his colleagues provided information and counseling for those with simple problems, and referred those with more complex issues to sex therapists. This model—information, counseling, and when necessary, referral—has informed Castleman’s sexuality journalism ever since.
In 1976, Castleman began writing for a tiny Marin County-based health magazine, Medical Self-Care, whose motto was: "It's not enough for doctors to stop playing God. The rest of us must get up off our knees." Medical Self-Care was one of the first health publications to cover both mainstream medicine and alternative therapies. Castleman became its editor in 1979. When the magazine folded in 1990, Castleman became a full-time freelance health journalist, writing for national magazines, among them:
AARP The Magazine, Family Circle, Ladies Home Journal, Mother Earth News, Mother Jones, Reader's Digest, Smithsonian, Yoga Journal .
From 1991 to 1995, Castleman answered all the sex questions submitted to the Playboy Advisor.
Castleman retired from magazine journalism in 2008 to devote himself to fiction, but continues to write about sexuality. He currently is publisher of the advice site GreatSexGuidance, and writes the “All About Sex” blog for PsychologyToday.com . He has also contributed to many websites, among them: Playboy.com, WebMD, and Salon.com.