In 1840, Trelawny was appointed Deputy-lieutenant of Cornwall and captain of the Cornwall Rangers; subsequently commanding the 2nd Cornwall Rifles Militia. In 1841 he was called to the bar, but never practiced and, in the same year, unsuccessfully contested Cornwall East Parliament Constituency as a Liberal. Salusbury-Trelawny sat as Member of Parliament for Tavistock between 1843 and 1852 and 1857 and 1865 and for Cornwall East between 1868 and 1874. He succeeded his father to the baronetcy in 1856 and retired from parliament in 1874 owing to "increasing infirmities".[3]