He studied at Redruth Grammar School and Emmanuel College, Cambridge, from which he received a lower second class degree[1] in modern languages.

McGrath and Mulville went on to write for shows such as Not the Nine O'Clock News and Alas Smith and Jones, and they were part of the team of writer/performers behind Injury Time (1980–1982),[4] and the Channel 4 comedy sketch series Who Dares Wins.

In 1981 McGrath wrote and performed in the radio show Glompus Van De Hloed's Tales From The Crypt alongside Andrew Sachs, Chrissy Roberts, Griff Rhys Jones and Mel Smith, from which an album was produced.

From 2006 to 2011, McGrath starred in the BBC's Three Men in a Boat series, alongside Dara Ó Briain and Griff Rhys Jones.

The series has included the trio rowing up the River Thames (similar to the eponymous 1889 novel Three Men in a Boat by Jerome K. Jerome), sailing from London to the Isle of Wight for a sailing yacht race, borrowing numerous vessels to make their way from Plymouth to the Isles of Scilly, taking to the Irish canals and rivers along with Ó Briain's dog (Snip Nua), travelling in the Mediterranean to Venice and attempting to find a boat to take to the anniversary of the Statue of Liberty, where in response to a challenge between Ó Briain and Rhys Jones (who had each secured a boat and refused to give it up to use the other's), he secured permission to use the Nantucket Lightship.

In the series McGuinness and McGrath embarked on a nationwide road-trip, "on a mission to explore Britain's sporting heritage by probing the hidden life of its towns and villages".

The series documented numerous arcane sports such as cheese rolling, toe wrestling and swamp soccer.

McGrath found out about his friend's death after failing to receive Cook's usual abusive phonecall the next morning.