He graduated from the law course of Tokyo Imperial University in 1912 and entered the Foreign Ministry.
In 1929 was awarded the degree of Doctor of Law for his Kokusaiho oyobi kokusai seiji yori mitaru Kokkai narabi ni Kumpu kaikyo no chii, a study of the Black Sea and the Bosporus and Dardanelles straits from the point of view of international law and politics.
He served in Russia, France, Turkey, and other countries, holding such posts as embassy secretary, foreign secretary, and embassy councilor. With the beginning of the Manchurian Incident in 1931, however, he found himself in disagreement with the government policy in Manchuria and he accordingly resigned and left the service of the Foreign Ministry.
In 1932 he was elected to the Lower House of the Diet, and thereafter continued to be reelected, holding office for a total of ten terms. During this period, he spoke out against the incursions of the military and Japan’s expanding military front, allying himself with the Seiyukai political party. He also held the position of president of The Japan Times, an English language newspaper.
In the postwar era, he served as minister of welfare in the Shidehara Kijuro cabinet in 1945. He joined with Hatoyama Ichiro in helping to form the Minshuto (Democratic Party), and in 1947 became president of the party. He adopted a policy of alliance with the Socialist Party, and was appointed foreign minister in the Katayama Tetsu cabinet formed the same year. After the resignation of the Katayama cabinet in 1948, Ashida formed a cabinet of his own with himself as prime minister and foreign minister. Like the Katayama cabinet, it represented a coalition of three political groups, the Democratic Party, the Socialist Party, and the Kokumin Kyodo (National Alliance). But it was subjected to frequent interference from the General Headquarters of the Occupation forces and w'as unable to function effectively. Meanwhile, a scandal involving the Showa Electric Company led to the arrest of Nishio Suehiro and other important cabinet ministers, and the cabinet resigned en masse. Ashida was also active in the formation of the Jiyu Minshuto (Liberal Democratic Party), but later became alienated from the mainstream of the party.