His uncle, also Charles Rainsford (died 1778), was deputy lieutenant of the Tower of London and used his influence to get him made second cornet in General Bland's 3rd dragoons in March 1744, a unit at that time active in the Flanders theatre of the War of the Austrian Succession.

The following year he was given a company to command under Prince Ferdinand of Brunswick in Germany, before re-joining Tyrawley as aide-de-camp, brigadier-general and chief engineer in 1762 to face the threatened Spanish invasion of Portugal.

Ordered home in 1763, with promotion second major in the Grenadier Guards and equerry to William, duke of Gloucester (1766–80), he commanded the army detachment at the king's bench prison at Southwark after the May 1768 riot.

He also served as MP for Maldon (1772–74) until his patron William Nassau de Zuylestein, 4th Earl of Rochford's nephew and heir was elected.

[citation needed] In 1782 Rainsford and Benedict Chastanier reached out to kindred Illuminist groups in Berlin and Paris by publishing a brochure in French about degrees of the Universal Society.