After making his NBL debut for the Coburg Giants in 1980 at the age of 18, Borner spent the 1981–1982 off-season attending Louisiana State University where he played college basketball for the LSU Tigers.
Borner played 518 NBL games over 22 seasons for four separate teams: Coburg / North Melbourne Giants, Illawarra Hawks, Geelong Supercats and Canberra Cannons.
[1][2] Borner won his only NBL Championship in 1989 as a member of the North Melbourne Giants who defeated the Canberra Cannons 2–0 in the best of 3 Grand Final series.
Borner also competed for the Boomers in four Summer Olympic Games: 1984 in Los Angeles, 1988 in Seoul, 1992 in Barcelona, and 1996 in Atlanta.
From around 1987 with the emergence of other players such as Mark Bradtke (208 cm), John Dorge (209 cm) and 7'2" (218 cm) Luc Longley, while still playing centre in a number of games, Borner also began playing at Power forward for the national team.