Horsnell attended Keebra Park State High School and captained the 1988 Australia Schoolboy team on a tour of New Zealand.
After six seasons and 82 matches for the Gold Coast club where the side finished last 3 years in a row in 1992, 1993 and 1994, Horsnell left in 1995, joining the new South Queensland Crushers franchise.
Horsnell left Parramatta at the end of the 1998 season, with his final game in first grade being a 15-15 draw against St George in Round 24 at Kogarah Oval.
On 23 October 2004, a charity match was held for Horsnell that featured the South Queensland Crushers and the Gold Coast Seagulls, both clubs by then defunct, being resurrected for the day.
The Australian quotes details from an affidavit in the matter outlining that Horsnell is taking action as a result of a medical assessment by a Gold Coast neurologist confirming that he is "almost certainly suffering from chronic traumatic encephalopathy".