Foreign Secretary David Cameron took his seat in the House of Lords after his official induction ceremony.
The former prime minister will now be known as Lord Cameron of Chipping Norton, reflecting his long-standing links with the Cotswolds town in his former constituency of Witney.
It was announced last week that he had been elevated to the Lords as a life peer to allow him to serve in Rishi Sunak's cabinet after he stood down as an MP in 2016.
He was previously in the Commons for 15 years, including five years as Leader of the Opposition and six years as Prime Minister.
He was supported by Lord True, the Leader of the House of Lords, and Baroness Williams of Trafford, the Chief Whip of the Lords.
Each of them was nominated for a peerage by Lord Cameron himself and Lady Williams had served as a minister in his government.
Lord Cameron, 57, wore traditional red robes for the short ceremony as he pledged allegiance to the king.
The red benches of the upper chamber were more crowded than usual, with people also sitting on the stairs and on the step at the foot of the throne.
Among those present was Lord Pickles, who served in the Cameron government.
Lord Cameron read the traditional oath: “I, David, Lord Cameron of Chipping Norton, do swear by Almighty God that I will be faithful and give true allegiance to His Majesty King Charles, his heirs and successors, according to law, so help me God.”
Shortly after leaving the room to mark the end of his speech, Lord Cameron returned to sit on the government front bench between Lord True and Lady Williams.
Peers were heard shouting to welcome him into the hall as he arrived.
However, Lord Cameron's appointment to the upper house was not universally welcomed, with the Lib Dems writing to the Prime Minister's ethics adviser calling on him to launch an inquiry into the appointment of the new Foreign Secretary.
Among her reasons for urging the inquiry, MP leader Wendy Chamberlain cited Lord Cameron's previous work as a lobbyist for investment firm Greensill Capital, for which he privately lobbied ministers in a bid to gain access to a emergency program for the coronavirus.
Ms Chamberlain said: “We need urgent clarity about David Cameron's financial interests, which could lead to serious conflicts of interest while he represents the UK on the world stage.
“If he was serious about acting with integrity, Rishi Sunak would have addressed these concerns by asking his ethics adviser to launch a full investigation into Cameron's appointment.
“David Cameron has serious questions to answer about whether he can act impartially in the interests of the British people. His judgment and integrity have been questioned throughout the last few years and for good reason.”
Meanwhile, the Lord Speaker on Monday used a speech at the London School of Economics to stress the importance of experience and independence in the House of Lords.
Lord McFall of Alcluith said ministers in the Lords often face a tougher grilling than those in the Commons.
He said: “Some are suggesting that ministers take an easy ride in the House of Lords. Let me tell you, nothing could be further from the truth.
“A former minister who served in both Houses told me that the experience of being questioned by peers is far more terrifying.
“In the Lords, the roasts are run by former secretaries and heads of the civil service, judges, ambassadors, European commissioners, former heads of bodies such as NATO or the Joint Intelligence Committee.
“These are people who know their subjects intimately and can get straight to the heart of any issue.”