After 20 years on the job, Howard Jenkins Jr., the first and only black to serve on the National Labor Relations Board, is stepping down today rather than be forced out by the White House.

The man whom some call "Mr. NLRB" because he served on the board for so long announced two weeks ago that he had asked President Reagan not to reappoint him to a fifth term. But Jenkins said in an interview this week that he wanted to continue serving on the board and decided to resign only because the White House indicated that it wanted him out.

"I've been around in Washington long enough to understand how Washington operates," said Jenkins, "and a few months ago I came to the conclusion that I wasn't going to be asked and therefore it was time to leave."

A White House spokesman denied that Jenkins was forced to retire. "We hadn't made any decision before we received his letter," the spokesman said.

But rumors that Jenkins, 68, wouldn't be reappointed had been circulating for months. He said he doesn't know why that decision was made. However, the Chamber of Commerce and other business groups indicated they would have opposed his nomination because they view him as being too liberal and too pro-labor.

Jenkins sees a certain irony in his departure. "I was sworn in as a member of the board the day after the civil rights march on Washington in August, 1963," he explained. "And I'm leaving office the day before the 20th anniversary of that march."

During that time, blacks have made "some progress" in the labor movement and the workplace, Jenkins said. "But I think the deeply rooted racism and sexism that pervades our total society is still here. . . . Many of the issues that prompted the 1963 march are the same issues being talked about today."

Jenkins said he had never felt discriminated against as a board member under six presidents, even though President Richard M. Nixon bypassed him when it was assumed that he would be designated chairman.

Jenkins said, however, that he felt he always was judged differently from other board members. "No matter what position one holds or how high a rung on a ladder he may reach, if he is black and American today, he will be judged on two bases--his competence and his race. I don't think that people should be misled into thinking that if a black gets on the board of directors of a company . . . that he ceases being a black person. The question of whether certain opportunities are opened to him depend upon a host of things because of his race."

A native of Denver, Jenkins, in 1941, became the first black man to pass the Colorado bar exam. That same year, he married Alice Elaine Brown, the first black woman to teach in the Denver public schools.

Jenkins volunteered to serve in the Army Judge Advocate General's office during World War II, but was rejected because he was black. Instead, he worked in the Denver Office of Price Administration and later in the local War Labor Board office. When the war ended, he was sent to Washington for 60 days to write a final report.

But he decided to stay in Washington, and joined the faculty of Howard University Law School, where he specialized in labor law and helped draft briefs in the court battles to desegregate the railroads and public schools, including the landmark Brown vs. Topeka Board of Education case. His work attracted the attention of several Labor Department officials, and he joined the agency in 1956. Three years later, he helped draft the Landrum-Griffin Act, sometimes known as the "employes' bill of rights."

Former labor secretaries Arthur J. Goldberg and Willard Wirtz urged President John F. Kennedy to appoint Jenkins to the NLRB in 1963.

"I think my presence helped the board discover blacks in the industrial work force," Jenkins said. "I don't think the board really had even thought about it to any marked extent until I came."

Organized labor and management also noticed and began sending black lawyers to NLRB hearings, Jenkins said. "I recognized that that person's presence wasn't necessarily essential to the position being taken by the litigant, but I think it gave black lawyers a great opportunity," he said. Then he added, "It didn't influence my actions, though."

In July, 1964, Jenkins wrote the majority opinion in a case that established the board's policy of refusing to help labor organizations that practiced racial discrimination.

Jenkins later wrote an angry dissent when the board ruled that a local union wasn't guilty of sex discrimination by having separate bargaining units for waiters and waitresses. A federal court agreed with Jenkins and overturned the NLRB decision.

A deeply religious man, Jenkins has had his share of criticism from both unions and management. But he scoffs at the critics. "By definition, half of the people who come before the board lose, and half of those who win feel they never should have come here in the first place, so no one is ever satisfied," he said. "And I've not changed my philosophy since I taught at Howard.

"It's extremely important to the preservation of our form of government and free enterprise system to maintain a strong free trade union," he said.

As of Monday, Don A. Zimmerman, an independent appointed in 1980 by President Jimmy Carter, will be the only person on the five-member board who isn't a Reagan appointee. Reagan is understood to be considering four black Republicans, one of whom is a woman, to fill Jenkins' seat.

Zimmerman will become the senior board member--something that concerns the departing member. "It may sound self-serving, but after I go, the senior member will have something less than three years' experience," Jenkins said. "I think the board needs a memory, . . . continuity."