[citation needed] Onslow briefly served as a Lord-in-waiting (government whip in the House of Lords) under the Earl of Beaconsfield (Benjamin Disraeli) between February and April 1880 and held the same position under Lord Salisbury between 1886 and 1887, and later served under Salisbury as Under-Secretary of State for the Colonies between 1887 and 1888 (in which post he was vice president of the first Colonial Conference in April 1887) and as Parliamentary Secretary to the Board of Trade between 1888 and 1889.

At 35, he was the youngest governor of New Zealand since George Grey was appointed in 1845 and the first since Robert FitzRoy in 1843 to have no previous experience in a vice-regal position.

The New Zealand government had recently cut allowances for the governor, and Onslow was able to obtain the position mainly as it was no longer attractive to more senior colonial administrators.

Onslow's twelve-year-old son and heir, Viscount Cranley, caught the disease and was at one time seen to be in danger of his life.

[1] In 1890 Onslow became embroiled in controversy over appointments to the Legislative Council (the upper house of the New Zealand parliament), which were made by the governor on the advice of his ministers.

Onslow, as a Conservative, is considered to have been sympathetic to the idea, especially as he was used to the system in Britain, where it was acceptable practice for an outgoing British government to nominate new members of the House of Lords.

Rumours of the "little list" began to spread and Onslow was made aware that, even if it was common practice for an outgoing government in Britain to nominate members for the House of Lords, this was not the case in New Zealand.

[citation needed] In 1903 he entered Balfour's cabinet as President of the Board of Agriculture and was sworn of the Privy Council the same year.

They had two sons and two daughters:[citation needed] Lord Onslow died at Hendon, Middlesex, in October 1911, aged 58.