The 43-year-old quit his role in Holyrood to take up a post as UK trade minister.
Anas Sarwar, the former Scottish Labour leader, has taken his seat in the unelected House of Lords.
Lord Sarwar, who led the Scottish party for more than five years, wore the traditional scarlet robes for the ceremony in the upper chamber.
He was supported by two former Scottish Labour MPs, Baroness Liddell of Coatdyke and Lord Robertson of Port Ellen, as he swore allegiance to the King on Thursday.
The 43-year-old was greeted with a handshake from the Leader of the Lords, Baroness Smith of Basildon, as he left the chamber.
Lord Sarwar resigned as Scottish Labour leader in July to don the ermine and become a trade minister.
He was among the first Labour politicians to demand Sir Keir Starmer resign as Prime Minister in a botched move in February, which initially saw the Cabinet rally around, giving Sir Keir a temporary stay of execution.
Following Sir Keir’s departure, Lord Sarwar was made a peer shortly after Andy Burnham entered No 10.
He has previously said he was “delighted and honoured” to have the chance to serve as trade minister.
“Throughout my life, I have only ever been motivated by public service,” he said. “The opportunity to serve our country is all I have ever asked.
“That’s why this is an opportunity I humbly accept.”
Lord Sarwar led Scottish Labour from 2021, a role he described earlier this year as the “hardest” in politics.
His leadership saw the party collapse to 17 seats in Holyrood.
First Minister of Scotland John Swinney previously accused Lord Sarwar of “hypocrisy” and an “appalling level of judgment” for taking the peerage.
“He said before the (Holyrood) election he had absolutely no interest in going to the House of Lords, he only wanted to be first minister of Scotland.
“Then, after the election, Anas Sarwar has accepted a place in the House of Lords – a body, by the way, he wants to abolish.”
The result of the race to replace Lord Sarwar as Scottish Labour leader is expected to be announced on September 19, with Joe Fagan and Michael Marra in the running.
Also taking her seat on the red benches was Baroness Ruth Mackenzie of Sherwood, the director of arts at the British Council, who has been made a Parliamentary Under-Secretary of State in the Department for Digital, Culture, Media and Sport.
