[1] Onslow's father, Arthur, was a politician, as was his maternal grandfather Thomas Foote, who had served as the Lord Mayor of London in 1649.

One of Onslow's first actions as a member of Parliament was to support the Exclusion Bill, which aimed, unsuccessfully, to deny the Catholic James II of England the British throne.

[3] Onslow was an active back-bencher during his early years in Parliament, and his increasing notoriety as a moderate Whig led to him being nominated for the position of Speaker in 1700 when the member for Surrey.

He proved to be a poor Speaker as he made no effort whatsoever to show any kind of neutrality, a fact which upset all but the most fervent Whigs.

When Onslow took the Commons to the House of Lords to hear their judgment on the case he challenged Black Rod on a trifling point of privilege, delaying the proceedings somewhat, which infuriated almost everyone in attendance.