His parents were living in Mawei at the time, where his father was employed in the ordnance factory there, but they had to evacuate to Hong Kong almost immediately after his birth because of the Sino-French War.
On his return to China, Cheng served from 1918 to 1927 in various legal capacities in the Beiyang government in Beijing, principally concerned with codification, trade mark, extraterritoriality and judicial reform, as technical expert of the Chinese Delegation to the Washington Naval Conference (1921), counsellor at the Ministry of Foreign Affairs, before becoming a judge of the Supreme Court of the Republic of China.
Cheng subsequently practised law in Shanghai from 1927 to 1932, the year when he acceded to the invitation of the Chinese Government to take up the post of Executive Vice-Minister and sometime Acting Minister of the Justice Ministry of the Republic of China, the capital of which had by then moved to Nanjing.
In 1935, he resigned and, while serving as an adviser in both the Justice Ministry and the Foreign Ministry, he was appointed by the Chinese Government as the Special Commissioner for the London International Exhibition of Chinese Art in England, which, as things turned out, provided a convenient excuse for moving the then most valuable treasures in the Forbidden City in Beijing to the south of China in order to escape the war that was then threatening and avoid being accused of being provocative.
From 1936 to 1945, Cheng served as a judge of the Permanent Court of International Justice in The Hague, Netherlands, the work of which was cut short by the outbreak of World War II.
In 1936, Cheng was decorated by the government of the Republic of China with the Order of Brilliant Jade, Class Two, with Grand Cordon, and elected a Fellow of University College London.