Rennie entered the Colonial Service in 1851, and was acting Colonial Secretary in the Falkland Islands from April 1856 until June 1857.[1] Rennie was in 1858 appointed Auditor-General of Hong Kong when the responsibility was split from Colonial Secretaries. He thereby received a seat of the Legislative Council of Hong Kong.[2] He served in these offices until 1870. Rennie in 1871 transferred to the post of Lieutenant Governor of St. Vincent.[3] Rennie was awarded the CMG in March 1874.[4]