Harmon began preaching in his church as a child. After attending seminary, he spent 13 years working for Americans United for Separation of Church and State as Associate Field Director for Religious Outreach, recruiting and training clergy willing to provide legislative testimony about issues of religion and government to legislative bodies.

In 2008, he began as a volunteer organizer of clergy working to gain marriage equality in Washington, D.C., serving as a steering committee member, simultaneously founding an inclusive church congregation and serving as Associate Pastor. He then co-founded Many Voices with Ann Thompson Cook, taking up the post of Minister of Welcoming Resources, later transitioning to co-director, then Executive Director.

In 2011, Harmon presented the lecture "Sexuality as a Gift or Problem: Re-educating the Black Church about LGBT Issues" at the Metropolitan Community Church of Corpus Christi's People of African Descent, Friends & Advocates Conference.

In 2012, Harmon was a presenter at the LGBTQ Victory Institute's annual OUT On the Hill conference.

In 2013, Harmon lectured at the University of Maryland on the topic of the Civil War and civil rights with his presentation "Creating a More Perfect Union."

In 2014, Harmon was a presenter for the workshop "Advancing Equitable Outcomes for Black LBGTQ Communities" at the Association of Black Foundation Executives' annual conference. He also joined dozens of faith and government leaders, including then-San Francisco Mayor Gavin Newsom, in petitioning San Francisco Archbishop Cordileone to cancel his appearance at the anti-LGBT extremist National Organization for Marriage’s rally and march. He also became involved with SAGE Story, a national digital storytelling and anti-isolation program for LGBT community elders.

In 2015, Harmon and his Many Voices co-founder Ann Thompson Cook published And God Loves Each One: A Black Church Guide to Sexual Orientation. The same year, he offered testimony before Washington, D.C. City Councilmember Yvette M. Alexander and the Committee on Health and Human Services in support of the Death with Dignity Act of 2015.

In 2016, Harmon was a panelist at Fordham University's LGBTQ panel discussion on race, religion, and sexuality. He was also a panelist at the Portland, Oregon's PFLAG Black Chapter screening of Holler If You Hear Me, a BET documentary about LGBTQ youth who are alienated from their families and churches. In addition, as part of Reconciling Ministries Network, Harmon traveled to Liberia as a trainer for the organization's African Christian Community and Theology Training. There, he led workshops on "human sexuality as it relates to the church and [the] community."

In 2017, Harmon spoke on the panels "Widening Our Circle Among Communities of Color" and "Into the Third Millennium" at the annual Rolling the Stone Away conference, a congress of LGBTQ+ Christian leaders. Also in 2017, he was named as one of "12 Faith Leaders to Watch in 2017" by the Center for American Progress, was selected for an Arcus Leadership Fellowship, and received the Distinguished Service Award from the Gay and Lesbian Activists Alliance of Washington, D.C.

In 2018, Harmon co-led "Bridging Divides," a faith leader social justice training workshop at the Radically Rooted: Summit for Change. He also presented at Souls A' Fire 6, a national conference on black queer theology, and led the workshop "Being in Relationship" at the Wild Goose Festival, a festival of "Spirit, Justice, Music, and Arts." The same year, as part of the Union of Affirming Christians, Harmon and dozens of other members condemned the Trump Administration for dismantling LGBT-friendly policies. Also in 2018, Harmon lectured at the Metropolitan Community Church of Washington, D.C. on "Becoming a Spiritual Force." In March 2018, the Metropolitan Community Church of New York honored Harmon at their Easter Awards Banquet and Celebration.

In 2019, Harmon acted as a trainer at the Unitarian Universalist Association's "Our Whole Lives Matter" facilitator training, a part of their faith leader development workshops.

In 2020 at the University of Redlands Graduate School of Theology's Applied Wisdom Institute, Harmon acted as a seminar facilitator for the Certificate in LGBTQ Leadership program.