Glenda was born and raised in the city of Leeds, West Yorkshire, England, specifically in the town of Guiseley. She commenced acting at an early age, soon landing major parts in local plays and pantomimes. Her break came in 1988 when she gained a part in the ITV soap Emmerdale Farm (1972) with her brother Craig McKay, playing on-screen siblings until Craig's TV death from a plane crashing into the village in Emmerdale Farm: Episode #1.1829 (1993). Glenda has starred in Ken Russell's The Rainbow (1989) and worked on Emmerdale until 1999. She was living in the Leeds village of Rawdon in 1998. As of 2021 she lives in Ilkley, Bradford, West Yorkshire, England where she's been living since 2012, and previously lived from 2002 to 2010. She is also a Year 6 teacher at Ben Rhydding Primary School in Ilkley. She married her husband Ken Cumberland in July 2003.