A dual-sport athlete, De Mattia represented Australia as a junior cricketer before choosing to focus on football in his top-age year.

Lauded for his leadership, Harry was appointed captain of the Dandenong Stingrays in his U18 year, where he averaged 19 disposals from 11 games. De Mattia also impressed for Vic Country.

As a prospect, De Mattia impressed with his 2-way running, contested ball winning ability and tackling.

Collingwood selected De Mattia with Pick 25 in the 2023 National Draft.

In December 2024, Harry signed a 2-year contract extension with Collingwood, making him a Magpie until at least the end of 2027.

Recruited from: Dandenong Stingrays/Edithvale Aspendale Junior Football Club