Walsh was educated in Dublin, receiving a BComm International from University College Dublin and an MA in Journalism from Dublin City University. Walsh started his career at The Sunday Business Post in 1998. A year later he won an Irish national media award for Social and Campaigning Journalism and moved to Kenya to work as a freelance journalist. Based in Nairobi, Walsh travelled widely across sub-Saharan Africa to report for The Independent of London and The Irish Times. In 2004 he joined The Guardian as the paper's correspondent for Afghanistan and Pakistan and moved to Islamabad, Pakistan. In January 2012 he moved to The New York Times as its Pakistan bureau chief. Following his expulsion from Pakistan, Walsh assumed the position of Cairo bureau chief for The New York Times after which he moved to Nairobi, Kenya, as the paper's Chief Africa Correspondent.