The federal Americans With Disabilities Act forbids any kind of employment discrimination against persons with disabilties. Employers with 15 or more employees must make "reasonable accommodations" for workers with disabilities. (42 U.S.C. § 12111.) The Act doesn't say whether or not this includes allowing an employee who has a disability to bring an assistance dog to work; the answer will depend on what is "reasonable" under the circumstances. The regulations interpreting the law say, however, that letting a blind employee use a guide dog at work is an example of a reasonable accommodation. (29 C.F.R. Part 1630, Appendix.)
Employees have sometimes had to go to court to force employers to make reasonable accommodations for them. For example, an Illinois physician, who was permanently injured in an accident but returned to work using a wheelchair, asked permission to bring a service dog to work with her at a federal Veterans Administration facility. Her doctors had recommended a service dog, because years of operating her wheelchair had placed extraordinary stress on her body. The VA refused; but a federal court ruled that the facility must allow the doctor to bring the trained service dog to work because it was a reasonable accommodation to her disability. (Branson v. West, 1999 WL 311717 (N.D. Ill. 1999).)
State laws that protect a worker with a disability from discrimination in the workplace may also extend to the worker's dog. In New York, for example, it is illegal to deny a qualified person a job or promotion simply because the person is accompanied by a guide, hearing, or service dog. (N.Y. Civ. Rights Law § 47-a; N.Y. Exec. Law § 296; see also Md. Ann. art. 49B, § 15; Minn. Stat. § 363.03; Mo. Rev. Stat. § 209.162; N.J. Stat. Ann. § 10:5-29.1; Nev. Rev. Stat. § 613.330; Tenn. Code Ann. § 8-50-103; Wash. Rev. Code § 49.60.180.) Many states make it unlawful for public employers, or those that receive state funding, to deny employment because of an assistance animal.