He enlisted in the Irish meteorological service (now known as Met Éireann) in 1971, and worked there until 2004, rising to the rank of Head of the Research and Training Division and later Deputy Director.

[4][5] In 2004, he moved to academia, becoming Met Éireann Professor of Meteorology at the School of Mathematical Sciences, University College Dublin.

[6] Shortly after formally retiring from UCD in 2011, he started writing a weekly mathematical blog called "That's Maths", about half of the articles also appearing in The Irish Times newspaper.

Lynch examined this issue first in a 2008 paper,[9] and later in his book The Emergence of Numerical Weather Prediction: Richardson's Dream.

"[10] In 2017 he received the Maths Week Ireland award for outstanding work in raising public awareness of mathematics.