 In 1810, he joined the 16th Regiment of Light Dragoons as a cadet and was Captain of the 5th Regiment of Dragoon Guards from 1816 and Major of the 55th Regiment of Foot from 1821.From 1818, Cust sat in Parliament as member of parliament for Grantham until 1826 and then for Lostwithiel from 1826 to 1832.As a Member of Parliament, he raised concerns about the management of public architectural projects, particular the works at Buckingham House.