McLynn was born in Sligo but grew up in Galway. She studied History of Art and Modern English at Trinity College, Dublin. She was heavily involved in Trinity's Dublin University Players (college drama society). She is married to the producer and agent Richard Cook. She supports both Aston Villa and Bohemian F.C.
Although only in her thirties when playing Mrs Doyle, makeup was used to make her look far older to fit the character's profile. She received a British Comedy Award for her performances in 1995. The award was presented to her by then-Leader of the Opposition, Tony Blair.
Other televised appearances have been in sketches on Bremner, Bird and Fortune, and numerous panel shows, including Just a Minute, Have I Got News For You, and If I Ruled the World. In 1999, she appeared in the film adaptation of Angela's Ashes. She has also appeared in Jennifer Saunders' Jam & Jerusalem. In the late 1990s she was recruited to play Mrs Doyle for a UK advert for online Tax returns by the Inland Revenue and became infamous for the annoying phrase 'Go Online'
Pauline also got an unusual opportunity during her only appearance on They Think It's All Over, the BBC1 TV show. Fellow guest panellist Gabby Yorath, requested that she be allowed to take part in the Feel the Sportsman Round (which was unusual at the time as the team captain usually did this along with the other, regular panellist). Gabby was granted her request, and Pauline (who was on the opposing team) was offered the chance to don the blindfold herself and take part with her team captain Rory McGrath. She was only the 3rd female to do so at the time.
McLynn was critically acclaimed for her performance in the 2005 film Gypo, receiving an Irish Film and Television Award nomination for Best Actress.
In 2009, she starred in the in ITV supernatural series Demons, playing psychic Karen Speedwell. At the end of 2009, she will return for the 3rd series of Jam & Jerusalem. On the 25th of August 2009 McLynn appeared on ITV1's Daily cooks challenge as the star judge.
In 2010 McLynn appeared in the Channel 4 series Shameless as Libby Croker.
McLynn is also a successful novelist. Her credits include 'Missing You Already' Bright Lights and Promises, Summer in the City, The Woman on the Bus, Right on Time, Better than the Rest and Something for the Weekend.