Chen moved to Hong Kong and established a private law practice in 1947. He was a founding member of the Hong Kong Bar Association in 1948 and served as its first secretary. During that time he lived in a luxurious mansion at Kowloon Tong. His daughter was sent to study in Switzerland. In 1949, he and some other pro-Communist intellectuals and professionals including Mok Ying-kwai and Wong San-yin founded the Hong Kong Chinese Reform Association in response to the Young Plan proposed constitutional reform suggested by the then GovernorMark Aitchison Young. The association demanded that all unofficial members of the proposed municipal council should be elected and the appointment system should be dropped. In a meeting on 13 July 1949 attended by about 400 delegates from 142 registered Chinese civic organisations, the association and the Chinese Manufacturers' Association of Hong Kong and also two Kowloon-based commercial bodies culminated signatures of 142 organisations which presented membership of 141,800 people from the business, industry, labour and education sectors in the Chinese community. After the constitutional reform was turned down by Alexander Grantham and London in 1952, Percy Chen contested for the two resumed elected seats in the 1952 Urban Council election. He was the most energetic campaigner and was expected to win. Chen declared "there is no other Colony where the system of Government is so archaic; where the system of nomination instead of election plays a bigger part in the selection of so-called representatives." He concluded that the "Democratic system of Government has not been developed in Hong Kong." He urged voters to treat the 1952 Urban Council election as a referendum on reform and show London that 90 percent of those eligible cared enough to turn out on Election Day. Chen eventually lost to Brook Bernacchi and William Louey in the election. He contested again in the 1953 Urban Council election but was still unable to win a seat. He and Mok Ying-kwai also tried to bring the comfort mission from Canton to Hong Kong in support of the Tung Tau Tsuen fire victims in 1951. The mission was rejected by the colonial government and Mok was subsequently deported in September 1952, Chen succeeded Mok as the chairman of the association and sought help from the Hong Kong Chinese Clerks Association in reorganising the association. The association remained one of the three pillars of the pro-Communist leftist organisations throughout most of the time in Hong Kong under colonial rule.
Marco Polo Club
In 1956, Chen founded the Marco Polo Club, a dinner club with a select membership consisting of mainly foreign businessmen, journalists, trade representatives and consular officers. It was the world's only social organisations in which Westerners could regularly and informally met the officials of the People's Republic of China, representatives from New China News Agency Hong Kong branch and Bank of China over French meals and whiskeys and sodas. There were no membership fees but the guest had to pay for their meal at the dinner meeting. Invitation reminder came in the form of a simple postcard mailed once a month to members. The card requested their presence at cocktails, a European-style formal dinner, and a screening of Chinese films on the last Thursday of each month at the private dining room at the Mandarin Hotel. Percy Chen did not allow Americans to dinner gatherings until 1972 due to the poor relations between the Communist China and the United States. Chen was also member of the Sino-British Club of Hong Kong and led a group to visit Canton in 1957. He was the member of the Hong Kong and Kowloon Committee for Anti-Hong Kong British Persecution Struggle during the 1967 Leftist riots against the British colonial rule.