Kerin kicked off in magazines in late 1995 working for Felix Dennis on the just-launched The Week, where he ran the business side of the magazine – particularly focussed on subscription growth. He launched MoneyWeek in 2001, and then ran Felix’s consumer magazines – including Maxim and its overseas expansion to become the world's best selling mens lifestyle magazine, plus Viz, Men’s Fitness, Bizarre and other fun things. The Week US launched 2001 and continues to this day. The Week became Britain’s fastest news growing magazine, recording 19 consecutive ABC increases and was the UK's best selling news magazine. Kerin launched The Week Junior in 2015, the UK’s largest children’s magazine and followed up with a US edition in 2020.

In October 2022 he founded Atlas, a strategic consultancy to help media clients grow their recurring revenue businesses; is clients include The Guardian, Haymarket, Google and The New Scientist. Atlas operates in the UK and US.