[2] He was appointed Governor of the Straits Settlements and High Commissioner for the Federated Malay States in 1920[7] and retired from the civil service in 1927.

During his administration, Guillemard was criticised by opinion pieces in The Straits Times remarking on his lack of "bold initiative."

Guillemard's attempts to decentralise more control over internal affairs to the Federated Malay States met with opposition, and some of his proposals were abandoned.

[14] Guillemard's obituary published in The Straits Times began by stating he was "not likely to be regarded by Malayan historians as a great Colonial Governor."

However it went on to call him "a sound administrator, and the right man for Malaya at the end of the first world war," praising him for his decentralisation efforts, reforms expanding the number of local seats on the Straits Settlements Legislative Council, and his financial stewardship during the Great Depression.