Liberal Democrat Jeremy Purvis, 39, was the MSP for Tweeddale, Ettrick and Lauderdale from 2003-11.
A former personal assistant to Lord Steel of Aikwood, the ex-Liberal leader, Mr Purvis will sit in the Lords as Lord Purvis of Tweed.
He was flanked by Lord Steel and the Advocate General for Scotland Lord Wallace of Tankerness as he swore the oath of allegiance to the Queen.
Also taking her seat in the Lords today was Conservative Baroness Hodgson of Abinger, a campaigner on women's issues and former chairman of the Conservative Women's Organisation.
Lady Hodgson's two supporters were her husband Lord Hodgson of Astley Abbotts and another Conservative, Baroness Seccombe.
They are among a group of 30 new peers announced by the Prime Minister in August.