MEXICO CITY — The United States ambassador to Mexico plans to resign from her post in May, according to a memo circulated on Thursday to embassy staff, the latest in a string of senior diplomatic departures from the region — and more broadly from the State Department.

The ambassador, Roberta S. Jacobson, 57, served just under two years in the post, after her arrival was delayed by a prolonged confirmation process. Analysts say her departure will be deeply felt by both American and Mexican officials — she was one of the most experienced Latin America experts in the State Department, having spent most of her 31 years there focusing on the region.

“I have come to the difficult decision that it is the right time to move on to new challenges and adventures,” Mrs. Jacobson wrote in her letter. “This decision is all the more difficult because of my profound belief in the importance of the U.S.-Mexico relationship and knowledge that it is at a crucial moment.”

The Trump administration has selected a nominee to fill Mrs. Jacobson’s vacancy, but has not yet released the name, according to an American official with knowledge of the decision who was not authorized to comment publicly and who discussed the matter on the condition of anonymity.