Williams, 21, completed the 2.4 mile swim, the 112 mile bike ride and full marathon in 14 hours, 22 minutes and 21 seconds.
That was enough to win silver in the 18-24 female category.
Williams was one of a host of Plymouth Tri Club members competing in the event.
Matthew McArthur was 218th overall and 30th in the men’s 30-34 section with a time of 11:53.05.
Other Plymouth Tri Club athletes to finish the gruelling competition were Ben Oman (12.29.39), Jamie Muirhead (12.37.14), Jason Hillier (14.51.05), Paul Lote-Williams (14.53.10) and Matthew Ash (15.41.50).
More than 1,500 athletes finished the event, one of the biggest Ironman races in Europe and offers qualification for the World Championships.