Langhorne A. (Tony) Motley, a key and controversial figure in shaping Reagan administration policy in Central America, resigned yesterday as assistant secretary of state for inter-American affairs to return to the private sector.

President Reagan nominated Elliott Abrams, assistant secretary for human rights and humanitarian affairs, to replace Motley.

Officials at several levels said Motley's departure after two years on the job does not signal any policy shifts. Motley, 47, was the architect of the October 1983 invasion of Grenada that stands as the administration's chief foreign policy success, and he is credited with a major role in forging an accord with Congress on El Salvador and bringing it within apparent range of agreement on Nicaragua.

"No assistant secretary of state for Latin America can point to more accomplishments than those achieved by Tony," said Secretary of State George P. Shultz in making the announcement at a press briefing. "He has been the architect and the general contractor and the chief mason of our policy in Central America . . . he never gives up. He keeps coming back."

Shultz said, "The president has come to admire him, as I have, as a real scrapper . . . we hope to get him back with us." Turning to Motley, he said, "So, Tony, go make a million bucks and then come back with us."

In a telephone interview, Motley said he had told Shultz he wanted to leave about four months ago "for personal financial reasons" and that Shultz asked him to stay through votes in Congress last week and until a successor could be confirmed.

"There is absolutely not anything to do with policy in this," Motley said. "I like the job, but it's been four years of public life and I know I should be focusing on other things."

Motley, who was born in Brazil of American parents and lived there until he was a teen-ager, moved to Alaska where he made a small fortune in real estate. He was named ambassador to Brazil in 1981 and took his present post in 1982.

Motley is known to have clashed with hard-liners in the White House over some of Reagan's tougher speeches, arguing that a more conciliatory line would go over better in Congress. But he said he had "no great disagreement" with those officials and did not blame them for last week's defeat by two votes in the House of a modified administration approach to Nicaragua.

"I wouldn't say it didn't work," Motley said. "A week before that vote you would not have predicted it would be that close."

Motley said that developing proposals for the Grenada action and then watching them be implemented "is something you can't forget." He said he had few regrets and that "there hasn't been one dull moment" over the past two years.

"Central America is heading in the right direction now," he said. He compared his post to a marathon relay race "in which the baton is passed along" and said that as he had built upon the work of his predecessor, Thomas O. Enders, "we've built some solid things to pass along, and Central America is one of those."

Motley said he had not yet decided among several business offers, some of them in the Washington area. A spokesman noted that Motley was "likely to participate actively in the upcoming campaign for governor" in Alaska, but Motley declined to comment.

Abrams, 37, has been chief of the human rights bureau for 3 1/2 years. He previously served as Shultz's assistant secretary for international organizations and earlier worked for the late Sen. Henry M. Jackson (D-Wash.) and for Sen. Daniel Patrick Moynihan (D-N.Y.).