Heller practiced employment law on behalf of employees and applicants for employment exclusively. In 1981 he joined Kator, Scott & Heller as a name partner and practiced with that law firm until early 1997, when he became one of the founding partners of Heller, Huron, Chertkof, Lerner, Simon & Salzman. Among his more important reported employment cases was Hopkins v. Price Waterhouse, a Title VII case which disapproved sexual stereotyping in judging female candidates for partnership and also established the respective burdens of proof for the parties in a mixed-motive case. There were five reported decisions in Hopkins spanning six years: 618 F. Supp. 1109 (D.D.C. 1985); 825 F.2d 458 (D.C.Cir. 1987); 490 U.S. 228 (1989); 737 F.Supp. 102 (D.D.C. 1990); 920 F.2d 967 (D.C.Cir. 1990).
Heller also litigated Sutherland v. Arthur Young & Co, 631 A.2d 354 (D.C. 1993) which upheld the right to recover punitive damages for employment discrimination under the District of Columbia Human Rights Act.; Brinkley-Obu v. Hughes Training, Inc., 36 F.3d 336 (4th Cir. 1994) in which the United States Court of Appeals for the Fourth Circuit applied the Equal Pay Act to uphold a claim by a woman that she had been paid less than men who preceded and followed her in jobs she held; and Anderson v. Group Hospitalization, Inc., 820 F. 2d 465 (D.C. Cir. 1987) in which the United States Court of Appeals for the District of Columbia Circuit upheld the use of both anecdotal and statistical evidence to prove discrimination in the promotion of African-American vs. white employees to supervisory positions within a major health insurer.
Heller also served as a volunteer lawyer for the American Civil Liberties Union in a series of cases challenging the excesses of the federal government loyalty and security programs. Most notable was the U.S. Supreme Court case of Cole v. Young , 351 U.S. 536 (1956). In Cole the Supreme Court held that the federal employee security program could not be used to remove or bar federal employees from non-sensitive jobs. In addition to ACLU work, Heller served as a court-appointed attorney in several appeals for indigent criminal defendants. Two of those cases were ultimately decided by the U.S. Supreme Court, Gore v. United States, 357 U.S. 386 (1957) and Greene v. United States, 358 U.S. 326 (1958). In Gore Heller served as appointed co-counsel with one of the great civil liberties lawyers in our country's history, Joseph L. Rauh.
In 1964, Heller was the Deputy Assistant General Counsel in the Department of Commerce. In 1965 he joined the War on Poverty as Assistant General Counsel for the U. S. Office of Economic Opportunity. He was the chief lawyer for the Head Start Program and then became the senior lawyer for the entire Community Action Program. In 1968, believing that President Johnson had decided to fight the wrong war (Vietnam) and abandon the right one (Poverty), Heller returned to general private practice, with the Washington office of a New York firm, Stroock & Stroock & Lavan. From 1971 to 1973 he also served as the elected Chairman of the American Civil Liberties Union National Capital Area affiliate, which he had helped found a decade earlier. As Chairman he organized the affiliate's legal challenges to the mass arrests and detentions of the Mayday 1971 demonstrators against the Vietnam War.
In addition to his ACLU involvement, Heller was a Board member of the Washington Lawyers Committee for Civil Rights and Urban Affairs, the Levine School of Music, and the Jenny McKean Moore Fund for Writers.