John E. Jacob, the soft-spoken insider who has been named to succeed Vernon Jordan as president of the National Urban League, said yesterday that he believes his organization's chief task in coming years will be to provide the basic services to poor and black people which government is not willing to do.

"At a time when people are without jobs, it is one thing to be out advocating and another to help them get a job," said Jacob, 46, who served for nearly three years as Jordan's chief deputy. "While we will continue to demand that the federal government play a major role, we will also try to get private industry to target resources to deal with our constituency."

Jacob was chosen Monday over better-known figures who were mentioned for the job, notably outgoing Atlanta Mayor Maynard Jackson. In the four years he spent as director of the Washington Urban League and the time he spent working under Jordan, Jacob developed a reputation as a low-profile technician who avoided the kind of personal celebrity that Jordan and other leaders have enjoyed.

"That's what I have been called on to do, but it does not mean that I cannot do other things," Jacob said of his nuts-and-bolts reputation. "I believe the League board felt that in order for our organization to address the kind of problems we are confronted with in the '80s, we needed somebody with thorough knowledge of the Urban League, how it functioned at the local level. I believe the struggle is going to be in the trenches."

He added, "I do not think there will be a difference in tone, but I am sure there will be a difference in style. I am not Vernon. I will not try to be Vernon."

The Urban League is the nation's oldest civil rights organization. It is an advocate of blacks and the poor and also has conducted surveys of minority attitudes and conditions. Through its local affiliates, it operates job training programs.

Del. Walter E. Fauntroy (D-D.C.), chairman of the Congressional Black Caucus, said, "I think the League made a very wise choice by elevating a tested and knowledgeable administrator who can effect the smoothest transition in leadership. There is very little time for on-the-job training."

Senior D.C. Superior Court Judge William S. Thompson, a member of the Washington Urban League board while Jacob was president of the local affiliate from 1975 to 1979, described him as an "excellent administrator" who left the organization in good financial and administrative shape.

"He was well-liked by the people who supported us financially," Thompson said. "That's what you need to replace Vernon. There may be only one Vernon Jordan, but I don't know of anybody else who was ready to step into his shoes."

As Jacob takes over the Urban League, the organization faces a threat to its lifeblood: the millions of dollars it receives every year from the federal government for performing such contracted services as job training. When the League's fiscal year began in July, it anticipated receiving $29 million over the next 12 months in U.S. contract work. Now, Jacob said yesterday, the League is projecting a cut of at least $6 million in that figure, due to Reagan administration budget cuts.

At the same time, he said, the organization's constituency needs even more help than before. "Our people are being damaged in all the major areas that we came to take for granted in the decade of the '70s," he said.

Jacob said a nationwide survey conducted by the Urban League last year showed that 24 percent of black heads-of-household were unemployed and that blacks felt unemployment to be their most pressing concern. The survey also indicated that 71 percent of black heads-of-household in families making more than $20,000 a year, middle-class by black economic standards, felt they suffered from racial discrimination.

He said he believes the Reagan administration's policies are fostering closer ties between middle-class, working-class and so-called "underclass" blacks than have existed for years.

"Middle income for blacks generally means two adults working in the same family," he said. "With the kind of pressures that exist now, what you see are one or both of those workers being confronted with unemployment. Our middle class is not so solid that families can afford to have somebody losing their job."

Perhaps the most publicized of Jacob's efforts as head of the local Urban League affiliate were two surveys the organization did in 1976 and 1978, both called SOS, Speak Out for Survival, in which Washington's poor were interviewed at random and their responses collected into authoritative portraits of poverty in the nation's capital.

Jacob is married and has one daughter, a senior at the Howard University School of Communications.