Professor Marcosson graduated from Yale Law School in 1986. After clerking for Judge George C. Pratt on the Second Circuit Court of Appeals, he joined the appellate staff at the Equal Employment Opportunity Commission in Washington, D.C., where he spent the next eight years briefing and arguing cases in the federal courts of appeals. During that time, Professor Marcosson also helped to design and conduct the EEOC's training program for its employees after enactment of the Americans with Disabilities Act before it went into effect in 1992.

Since joining the faculty, Professor Marcosson's research and writing has concentrated on constitutional law (especially the Fourteenth Amendment), and the civil rights issues facing lesbians, gay men, bisexuals, and transgender people. He has served on the Board of Directors of the National Lesbian and Gay Legal Association, and was the programming coordinator for its annual conference in 1998. From 2008-2014, Professor Marcosson served on the Coordinating Committee of the Fairness Campaign, Louisville's long-standing LGBT civil rights organization. In 2013, he began service on the Kentucky State Advisory Committee for the United States Commission on Civil Rights.

In 2002, Professor Marcosson published the book Original Sin: Clarence Thomas and the Failure of the Constitutional Conservatives. It takes a critical look at the record of the Supreme Court's most conservative members, examining whether they have been consistent in applying their "originalist" method of constitutional interpretation, especially in the landmark case deciding the presidential election of 2000.

From 2004-2006, he served as the School of Law's Associate Dean for Student Life. From 2005 through 2011-12, Professor Marcosson chaired the law school's Admissions Committee.