After the establishment of the People's Republic of China in 1949, the government adopted the pinyin transliteration method and used it to write proper names using the Latin alphabet. In theory, that's when Peking became known in the west as Beijing.

Why did Peking become Beijing?

The occupying Japanese in 1937 imposed the name Peking (Beijing), then with their surrender in 1945, the Nationalist Government restored "Beiping". ... As noted above, the pinyin romanization, "Beijing", was adopted for use within the country in 1958, and for international use in 1979.

When did Peking become known as Beijing?

multitude of names across their very long histories. But English-speakers have been remarkably slow at adapting to their Chinese names. To English speakers, Beijing was “Pekin” or “Peking” until about 1880, when “Peking” took off; similarly, Nanjing was “Nanking” or “Nankin.”