U.N. Ambassador Andrew Young abandoned the traditional partisan neutrality of envoys and injected himself into New York City's turbulent Democratic primary tonight by sponsoring a fund-raising reception for David Dinkins, the city clerk and candidate for Manhattan borough president.

It was young's first political foray in New York since becoming the U.S. representative to the United Nations and it placed him, in effect, against Democratic contender Robert F. Wagner Jr., whose father, a former mayor, was the last white borough president of Manhattan before the job came to be regarded by political lenders as a black domain.

Dinkins, who is black, is also opposed by State Assemblyman Andrew J. Stein and Ronnie M. Bridge, a former aide to Mayor John V. Lindsay both of whom are white. They are seeking the seat held by incumbent Borough President Percy E. Sutton, who is challenging Mayor Abraham D. Beame in the September mayoral primary.

The $100-per-person fund-riser, billed as "An Evening with Andrew Young," and held in the Tower Suite of the Time-life Building in midtown Manhattan, stemmed from a friendship dating back to undergraduate days at Howard University, according to an aide to the ambassador.

Thomas Offenberger, the aide, said Young did not regard his sponsorship of the reception as unusual, saying, "Mr. Young is a politician as well as ambassador. He intends to remain active in polities , to the extent that his time permits."

Offenberger said Young sponsored the reception only in the sense of lending his name to the event, and that Binkins' campaign staff made all of the arrangements and rented banquet space in the Time-Life Building.

Dinkins, a former state assemblyman, was nominated for deputy mayor in 1973 by Beame, but he withdrew following disclosures that he had failed to pay federal, state or city personal income taxes for four years.

While not unheard of, the overt involvement, of ambassadors in local partisan politics is relatively unusual in the United Nations.

Young was scheduled to make a brief handshaking appearance at the reception tonight. Offenberger said he did not know what future campaign assistance the ambassador would offer before the primary.