Mannion was born in 1958 in Sligo, Ireland, one of eight children (four brothers and three sisters). He attended school at St. John's Boys School and Summerhill College, in Sligo. He went on to Trinity College, Dublin, where he graduated with a Bachelor of Business Studies in 1979. When he left college, he first worked with Ulster Investment Bank for a few years, before moving to Emirates Airlines in 1987. In 2005, while President, Group Support Services with Emirates, he joined Aer Lingus as CEO.
He joined Royal Brunei Airlines in 2011 on a five-year contract as Deputy Chairman.