The eldest of four children, his parents own the Robin Horneff Performing Arts Center in Waldwick, New Jersey while his father, Van, also owns a gymnastics mat business. He grew up in Saddle River, New Jersey. Horneff began acting professionally in 1992 at the age of thirteen. He made his Broadway debut in John Guare's Four Baboons Adoring The Sun opposite Stockard Channing and James Naughton.
His first film appearance was a relatively minor role in the cult film The Sandlot as Phillips, the arch-rival of the title characters. This was soon followed by an appearance in Law & Order (in which he played a 14-year-old defendant who brutally murdered another youth) as well as leading roles in films such as Ghost in the Machine and Born to Be Wild.
In 1995, he won the awards for Best Lead Actor in a Made for TV Movie at the Youth in Film Awards and Best Young Leading Actor - Feature Film at the Young Artist Awards for The Yearling and Born to Be Wild respectively.
After several other television and film appearances, most notably the acclaimed Emmy Award-winning mini-series Oldest Living Confederate Widow Tells All, he temporarily left acting to finish high school and later to attend Columbia University in New York City where he received his degree in English.
He also travelled extensively during this time, visiting France, Spain and Germany. His family often visit relatives in Europe, which has allowed him to learn to speak both French and German fluently. He also spent several months in Russia, helping underprivileged children in and around Kostroma .
In 2002, he made his acting comeback with a second guest appearance in Law & Order. Since then, he has appeared in numerous different series such as The Handler, CSI: NY and House in addition to several horror films such as The Roost. He currently resides in Los Angeles.
Outside of acting, he is a keen athlete, having competed in three events in the Junior Olympics when he was younger. His hobbies include tennis, basketball, karate (in which he has a black belt), skiing, rock-climbing, Brazilian Jiu-Jitsu and flying. He took flying lessons from the age of fourteen and obtained a pilot's license several years later .