De Smith's research and writing in the field of constitutional law focused in particular on the constitutional problems of developing countries: his advice in this area was frequently sought by the United Kingdom and other governments, and his work led to the publication of The New Commonwealth and its Constitutions in 1964, and after a stay as visiting fellow at the Center for International Studies and as visiting professor at the Law School of New York University, of Microstates and Micronesia in 1970. To the end of his career he continued to be active as a consultant upon the constitutional problems of emergent states and nations. A second area of interest was administrative law. From the publication in 1959 of the first edition of Judicial Review of Administrative Action, his reputation rapidly became established. He produced two further editions of this work in 1968 and 1973. A fourth edition was prepared in 1980 by Professor John M. Evans. Two subsequent editions of de Smith's ground-breaking work have been written by an editorial team led by Lord Woolf, Professor Jeffrey Jowell QC and Professor Andrew Le Sueur. Among his other works was a popular student text, Constitutional and Administrative Law, with subsequent editions prepared by Barbara de Smith, Professor Harry Street and Professor Rodney Brazier. de Smith was joint-editor of "Commonwealth and Dependencies" in the third edition of Halsbury's Laws of England and editor of "Administrative Law" in the fourth edition of that work. An obituary in the Cambridge Law Journal paid tribute to "a legacy of outstanding scholarship. He reshaped administrative law as an academic subject in the United Kingdom, and his wide-ranging contributions to the literature of public law were consistently incisive and constructive. His style was both elegant and distinctive: like Blackstone – as Jeremy Bentham put it – he spoke the language of the scholar and the gentleman". The Cambridge Law Journal noted that de Smith "was at his happiest in postgraduate teaching and the supervision of research students, but he cared deeply about all aspects of teaching and tripos reform... Those that knew him will remember him as a somewhat reserved person with a quiet sense of humour, though they will not have known that the appearance of reserve was the result of deafness caused by his artillery service in the war; he was invariably encouraging to his students and younger colleagues and he was generous in his assessment of others". A notice in the Modern Law Review, a journal for which he served as secretary for many years, gave the following assessment:
His work in administrative law has been of seminal significance in the development of the principles of judicial review by courts throughout the Commonwealth. With characteristic modesty, he was astonished by the success of his major books. ... Stanley de Smith was a scholar and legal writer of exceptional quality. He was shy and gave a—perhaps deceptive—impression of diffidence, but his conversation was enlivened by a dry humour, sometimes rather reminiscent of his hero, Maitland, with whom as a constitutional lawyer he will certainly stand comparison
