Cathcart graduated from Richard Stockton State College (New Jersey) in 1976 and the Harvard Graduate School of Education in 1978. He received his J.D. from Northeastern School of Law in 1982.

Cathcart was a staff attorney at the North Shore Children's Law Project. He has served on the boards of the National Lesbian and Gay Law Association, the Civil Liberties Union of Massachusetts, Community Works, and the National Committee for Responsive Philanthropy.

A longtime leader in the lesbian and gay community, Cathcart served from 1984 to 1992 as executive director of Gay & Lesbian Advocates & Defenders (GLAD) in Boston, New England's lesbian, gay, and AIDS legal organization.

Executive director of Lambda Legal since 1992, Cathcart is a leading strategist and spokesperson in the movement to achieve full recognition of the civil rights of lesbians, gay men, bisexuals, the transgendered, and people with HIV or AIDS.

Cathcart has made Lambda Legal an unparalleled national force through its far-reaching litigation and public education. Under his leadership, Lambda Legal promotes security, respect, and fairness for lesbian and gay youth, families, and all people regardless of sexual orientation or HIV status.

Cathcart has fostered dramatic growth in the organization as well as in the scope of its work. He oversaw the opening of Lambda Legal's Midwest Regional Office in Chicago in 1993 and Southern Regional Office in Atlanta in 1997. In 2002, Lambda Legal opened a South Cental Regional Office in Dallas. From its regional offices, including the Western Regional Office in Los Angeles and from its New York Headquarters, Lambda Legal responds to requests for help and information from thousands of individuals facing discrimination each year.