He returned to his original regiment, the Foot Guards, on 27 March 1759 with the rank of captain-lieutenant and was promoted lieutenant-colonel on 7 November 1759.

As a reward for his support of the Rockingham administration, he received the post of Out-Ranger of Windsor Forest for life in 1765.

In the following year, Onslow took a leading role in the efforts to block printers from reporting debates in the Commons.

The resulting confrontation, fueled in turn by a report that described him as "little cocking George" (an allusion to his enjoyment of cockfighting), was a severe blow to the prestige of the North ministry, and Onslow was hanged in effigy on Tower Hill alongside an effigy of Sir Fletcher Norton, speaker and fellow-member for Guildford.

Onslow remained a steadfast supporter of North, opposing efforts to make peace after the surrender of Burgoyne at Saratoga.