He served as a special agent for the Federal Bureau of Investigation in 1933 and 1934, the highlight of his career there being his participation in an unsuccessful trap set for famed gangster John Dillinger. He was then Connecticut director of the National Youth Administration from 1935 to 1938. He was assistant to five successive United States Attorneys General (Homer Cummings, Frank Murphy, Robert Jackson, Francis Biddle, and Tom Clark) from 1938 to 1945.
As a special agent for the Attorney General, Dodd was basically a trial-level federal prosecutor. He worked primarily on criminal and civil liberties cases, including the prosecution of the Ku Klux Klan in the 1930s. In 1942 he was sent to Hartford to prosecute a major spy ring case in which five defendants were accused of violating the Espionage Act of 1917 by conspiring to gather and deliver U.S. Army, Navy and defense information to Germany or Japan. Four of the five pleaded guilty, but Dodd tried and won the conviction of the fifth, Reverend Kurt Emil Bruno Molzahn.
Dodd became vice chairman of the Board of Review and later executive trial counsel for the Office of the United States Chief of Counsel for the Prosecution of Axis Criminality at Nuremberg, Germany, in 1945 and 1946. He practiced law privately in Hartford, Connecticut, from 1947 to 1953.
Dodd was very involved with the trials and had a reputation for his tough cross examinations. Dodd cross-examined defendants Wilhelm Keitel, Alfred Rosenberg, Hans Frank, Walther Funk, Baldur von Schirach, Ernst Sauckel, and Arthur Seyss-Inquart. In addition to cross examining, Dodd drafted indictments against the defendants, showed films of concentration camps, provided evidence of slave labor programs, and presented evidence of economic preparations by the Nazis for an aggressive war.
Dodd showed through his evidence that Ukrainian Overlord Eric Koch and defendant Polish Overlord Hans Frank were responsible for the plans to deport one million Poles for slave labor. Dodd also showed evidence that defendant Walther Funk turned the Reichsbank into a depository for gold teeth and other valuables seized from the concentration camp victims. Dodd showed a motion picture of the vaults in Frankfurt where allied troops found cases of these valuables, containing dentures, earrings, silverware and candelabra. Dodd had a flair for drama and showed many gruesome items of evidence, such as a shrunken, stuffed and preserved human head of one of the concentration camp victims that had been used as a paperweight by the commandant of Buchenwald Concentration Camp.
Final pleas were made on August 31, 1946, and the Tribunal announced its judgment in September 1946. Dodd assisted the Allied prosecuting team of convicting all but 3 of the defendants. Twenty of the 21 Nazis had claimed innocence, including Hermann Goering, whom Dodd had charged with ordering Reinhardt Heydrich to kill the Jews. In addition to prosecuting the individual defendants, Dodd demanded in his summation to the Tribunal that all 6 of the indicted Nazi organizations be convicted of crimes against humanity, on the same grounds of the crimes against humanity ascribed to the individual defendants. These six organizations are the Leadership Corps, the Reich cabinet, the Gestapo, The Storm Troops (SA), the Armed Forces, and the Elite Guard (SS). Dodd felt that these organizations should not escape liability on the grounds that they were too large, part of a political party, etc.
Dodd was given several awards in recognition of his work at the Nuremberg trials. Justice Jackson awarded him the Medal of Freedom in July 1946 and President Truman awarded him the Certificate of Merit, which Justice Jackson personally delivered to him in Hartford in the fall of 1946. Dodd also received the Czechoslovak Order of the White Lion. In 1949, the Polish government had intended to award Dodd with a badge of honor called the Officer's Cross of the Order of Polonia Restituta, but Dodd rejected the medal due to his commitment to human rights and views that the Polish government was imposing a tyranny similar to that imposed by the Nazis, and accepting an honor from the President of Poland would be like accepting one from the Nazis.
Before becoming a U.S. senator, Dodd was hired to lobby for Guatemala in the United States for $50,000 a year by Guatemalan dictator Carlos Castillo Armas.
As chairman of the Senate Subcommittee on Juvenile Delinquency, Dodd worked to restrict the purchase of mail order handguns, and later shotguns and rifles. These efforts culminated in the Gun Control Act of 1968, which Dodd introduced.
Recently, with the discovery and publication of a letter from the Library of Congress to Senator Dodd from July 1968 confirming his request to have a series of Nazi laws translated from documents provided by Senator Dodd, controversy has arisen as to how much of the Gun Control Act of 1968 was taken directly from gun registration laws enacted and enforced by the Nazis in Germany and areas under Nazi control. It is believed that Senator Dodd obtained copies of the Nazi laws while serving as a prosecutor at Nuremberg.
In 1967 he was censured by the Senate for using campaign funds for personal purposes. In 1970, after suffering a heart attack, he chose not to run for re-election for the Senate, and the Democrats nominated Joseph Duffey to replace him. However, Dodd entered the race after the primary as an Independent. Taking just under a quarter of the vote, both he and Duffey lost to Lowell Weicker.