in 1951 he joined the firm of paul, weiss, rifkind, wharton & garrison, in his first position as an associate lawyer. he became a partner of that firm in 1960. in 1962, costikyan was elected chairman of the new york county democratic committee, defeating carmine desapio, and served two years. he was credited for removing tammany hall influence, thus reforming the democratic party and bringing it into the 20th century. he was abraham beame's campaign manager in the 1965 mayoral campaign. he served on commissions investigating the new york city government for governor nelson a. rockefeller and mario m. cuomo. he dropped out of his campaign for mayor in 1977, but instead joined the campaign of edward i. koch, formerly a political adversary. for many years until his death, he served as a member of the advisory board for the center for new york city law at new york law school.