Jennings was born in Fort Lauderdale, Florida. He was the youngest of five children to Chester Henry, an itinerant Southern Baptist preacher, and Alice Verna (Johnson) Jennings. His family was poor and constantly moved around the South as his father sought a permanent post. His father died when Kevin was eight and the family was living in a Lewisville, North Carolina trailer park. From then on he grew up in a rural atmosphere that was intolerant of African Americans and gay people; several of his cousins and uncles were in the Ku Klux Klan. He was constantly taunted and bullied.
He attended Paisley Magnet School in Winston-Salem, North Carolina where he did well academically, but was beaten by classmates for what they saw as effeminate behavior and attempted suicide after realizing he was gay. "The first day of 10th grade I actually refused to go back to school because I simply wasn't going to go back to a place where I was bullied every day." After he and his mother moved to Hawaii he graduated from Radford High School in Honolulu.
Jennings then attended and received a bachelor's degree magna cum laude in history from Harvard University, where he delivered the Harvard Oration at the 1985 commencement.
He became a high school history teacher, first at Moses Brown School in Providence, Rhode Island, from 1985 to 1987, and then at Concord Academy in Concord, Massachusetts, from 1987 to 1995, where he was chair of the history department. In 1992 the Edward Calesa Foundation named Jennings one of fifty "Terrific Teachers Making a Difference". Most of his students accepted him when he revealed his sexual identity after years of keeping it secret.
While at Concord Academy in 1988, Jennings started the nations' first gay-straight alliance together with a female student. Jennings then co-founded the Gay and Lesbian Independent School Teacher Network (GLISTeN) in Boston in 1990, to address the problems facing GLBT students. It held its first conference the following year, when it changed its name to the Gay and Lesbian School Teachers Network (GLSTN). The organization started out as a small local one but gained a strong supportive reaction. In 1992, Jennings was appointed by Governor William Weld to co-chair the Education Committee of the Governor's Commission on Gay and Lesbian Youth. He was the principal author of, "Making Schools Safe for Gay & Lesbian Youth", a commission report. The Massachusetts State Board of Education adopted the report as policy in May 1993 and the state became the first in the U.S. to outlaw discrimination of public school students on the basis of sexual orientation in December 1993.
In 1993, Jennings was named a Joseph Klingenstein Fellow at Columbia University's Teachers College, from which he received his master's degree in interdisciplinary studies in education in 1994. In 1994 he wrote Becoming Visible: A Reader in Gay & Lesbian History for High School and College Students, the "first book of its kind" for a high school audience. Jennings moved GLSTN to New York to accompany his studying, and decided to make the organization national in scope. In doing so, he also changed its name to the Gay, Lesbian and Straight Education Network (GLSEN), to give it a broader focus. The organization seeks to end discrimination, harassment, and bullying based on sexual orientation and gender identity. In both 1995 and 1996 Jennings was in Out magazine's list of "Top 100 Newsmakers and Earthshakers".
In 1997, Newsweek magazine named Jennings to its "Century Club" of people likely to make a difference in the 21st century. Jennings earned an M.B.A. from New York University's Stern School of Business in 1999. By that year, GLSEN was headquartered in the Chelsea, Manhattan neighborhood of New York City and had a staff of 18 and budget of $2.5 million.
In 1998 he won the Lambda Literary Award in the Children's/Young Adult category for his book Telling Tales Out of School. He has published six books on gay rights and education. His works have described his own past as a closeted gay student.
In July 2004, Jennings received the National Education Association (NEA)'s Virginia Uribe Award for Creative Leadership in Human Rights. NEA Republican Educators Caucus chairwoman Diane Lenning protested the award because—by her reading of a story in Jennings' book One Teacher in 10—she thought he broke Massachusetts law in 1988 by not reporting a sixteen-year-old gay high school student's relationship with an older man. Three days later, the caucus ousted Lenning as chairwoman over her stance against gays, and later that month The Washington Times published a letter from Jennings saying the accusations were hurtful, inaccurate and potentially libelous. CNN subsequently confirmed that the student was above the age of consent in Massachusetts and not sexually active. The incident resurfaced in 2009 as part of a social conservatives' campaign against Jennings' appointment to head the U.S. Department of Education's Office of Safe and Drug-Free Schools.
Jennings is a lifelong, avid ice hockey fan, who has played in the New York City Gay Hockey Association. In 2005, he suffered a near-fatal heart attack after a game, but he recovered and returned to the ice in 2007. In 2008, Jennings spoke out against the practice of homophobic chants from fans at New York Rangers home games, and stopped his practice of regularly attending their games for about a month. Jennings and the director of the Gay Hockey Association met with officials of the Rangers and Madison Square Garden but failed to get much action from them.
Jennings stepped down as head of GLSEN as of August 2008. By then, GLSEN had two regional offices and a staff of 40, and there were gay-straight alliances in over 3,700 schools registered to GLSEN.
On May 19, 2009, Obama administration Secretary of Education Arne Duncan announced Jennings' appointment as an Assistant Deputy Secretary for the Office of Safe and Drug-Free Schools, starting July 6, 2009 as the third director of the office, which was established in 2002 during the George W. Bush administration pursuant to the Safe and Drug-Free Schools and Communities Act part of the No Child Left Behind Act of 2001.
Social conservatives campaigned against Jennings' appointment because they alleged he condoned child molestation based on the 2004 incident over a teen's story he related in his book One teacher in 10: LGBT educators share their stories. The allegations were proven to be false when it was shown the student was above the age of consent and no sex had occurred. 53 Republican members of the House of Representatives signed a letter to the Obama administration that called for Jennings' dismissal. Education Secretary Duncan, the White House, the NEA, and the National Association of Secondary School Principals have supported Jennings' appointment, with Duncan saying Jennings was “uniquely qualified for his job.”
As Assistant Deputy Secretary, Jennings has focused on matters relating to teacher safety, classroom discipline and bullying. His office has awarded safety grants worth millions of dollars. In August 2010 his office hosted the first-ever National Bullying Summit which he helped organize. In September 2010 Jennings became one of the notable members of the National Action Alliance for Suicide Prevention (NAASP), a public-private partnership designed to advance and update the 2001 National Strategy for Suicide Prevention and an outgrowth of the Suicide Prevention Resource Center. The NAASP will initially focus on three high-risk populations; LGBT Youth, American Indians/Alaska Natives, Military/Veterans.
On May 19, 2011, the Boston-based nonprofit organization Be the Change, based in Cambridge, Massachusetts, announced that Jennings would be resigning his position with the Obama administration and on July 25 he would become President and Chief Executive Officer of the organization.