The Sixers also said Thursday that Ed Stefanski will stay as general manager. Stefanski had held both roles since replacing Billy King in December 2007. King succeeded Thorn as New Jersey's GM last month.
Thorn was president and general manager of the Nets for 10 years before leaving in early July. Thorn and Stefanski worked together in New Jersey and helped lead the franchise to consecutive trips to the NBA finals. The Nets lost both times.
"There's been a lot of speculation that I was retired," Thorn said. "I was never retired. I retired from the Nets."
Sixers chairman Ed Snider says the organization was excited to add one of the NBA's "top executives." The Sixers went 27-55 last year and missed the playoffs for the first time in three seasons.
Thorn was contacted by Snider only a week after he left the Nets.
Thorn had called the shots for the Nets since 2000 and hired Avery Johnson as coach before resigning. Thorn helped turn the Nets into a contender in 2001 when he engineered a trade to bring Jason Kidd to New Jersey.
Before joining the Nets, Thorn was the NBA's executive vice president of basketball operations from 1986-2000.
This is the latest big move this summer for the Sixers after they missed the playoffs last year. They fired coach Eddie Jordan after one season and replaced him with Doug Collins. Then they traded disgruntled center Sam Dalembert and drafted Ohio State guard Evan Turner with the No. 2 overall pick.
"We could always use some more, but I like some of the pieces that we have," Thorn said.
Stefanski said after the season the Sixers took an "unacceptable" step backward in finishing 27-55 and missing the playoffs for the first time in three seasons.
"We don't want to take any more steps backward," Thorn said. "We want to take steps forward."