He swam at the Worcester Boys Club, and began attending prestigious Williston Academy Preparatory School in Easthampton, on a scholarship at age 13.

In August 1954, he swam breaststroke with the team of backstroker Bill Sonner, Butterflier Dick Fadgen, and Freestyler Dave McIntyre and won the 400-meter medley relay at the National AAU Swimming meet in Indianapolis with a time of 4:34.7.

[9] In an equally rare distinction, in 1954 he captured the world record in the 400-individual medley with a time of 5:31.2, set at the Dixie American Athletic Meet in Kinston, North Carolina.

[5][3] He continued to pursue swimming at Wilmington's Central Branch YMCA and coached some with Hercules, Inc., summer club teams.

Mattson used the majority of his savings to begin building the Wilmington Swim School, and quit his textiles job by 1963 to coach full time.

The $300,000 cost was met by draining his life savings, selling around $40,000 in debenture bonds,[4] contributions from family members and a favorable land deal.

[3] Recognized by the U.S. and internationally as a skilled coach, in 1975 he assisted the Panamanian swim team in preparing for the Pan American Games in Mexico City.

Mattson credited the win partly to teaching body-mind synergistics to the team, and improving flexibility by conducting hatha yoga sessions taught by his wife Nancy who also served as a coach.

[3] Swimmers he mentored at Wilmington included 1968 Olympians Dave Johnson and Jenny Bartz, 1976 Olympian and NCAA Champion Steve Gregg, and national champions, Marry-de Mackie, Jenni Franks and Seth Van Neerden, Tom Evans (NCSU) and Eric Ericson (UNC).

[4] Mattson's former swimmers who became successful coaches included Edgar Johnson, older brother of the 1968 Olympian Dave, and Sid Cassidy.