Baroness Cox the longest serving Life Peer

 

 According to Wikipedia, the longest-serving life peer is Baroness Cox (Crossbench), who is also the longest-serving female member of the House of Lords. She first took her seat on March 2, 1983.  [16]

From Hansard - Caroline Anne, Mrs. Cox, having been created Baroness Cox, of Queensbury in Greater London, for life—Was, in her robes, introduced between the Lord Trefgarne and the Lord Beloff.Baroness Cox.

Volume 439: debated on Wednesday 2 March 1983