He matriculated at Christ Church, Oxford on 17 December 1680, aged 17 and was awarded BA in 1684.
[2] Mordaunt joined the army and was an ensign by 1689 and captain of the 1st Dragoon Guards by 1693.
Another regiment of marines was raised under his colonelcy on 10 March 1702, which was converted to infantry in May 1703 and disbanded in July 1713.
At the 1708 British general election he transferred to Richmond, Yorkshire and was returned as MP for the borough.
[2] At the 1715 British general election Mordaunt was again returned as Whig MP for Richmond by his cousin, Lord Wharton.