He served as Bart Starr's back-up in Green Bay for four seasons, although he was in the U.S. Army in 1968 and wasn't activated until hours before the season finale in mid-December. Horn relieved injured back-up Zeke Bratkowski in the first quarter and guided the Packers to a one-point win over the Chicago Bears at Wrigley Field, which denied them the division title.
Horn's greatest game as a professional came in 1969 at Lambeau Field. Playing at home in the season finale on December 21 against the St. Louis Cardinals, he completed 22 of 31 passes for 410 yards, with five touchdown passes and one interception. He started five games in 1969, leading the Packers to a 4–1 record and throwing for 1,500 yards, 11 touchdowns and 11 interceptions. Green Bay finished at 8–6, third place in the Central division, four games behind the Minnesota Vikings (12–2), who clinched the division title in November.
Horn finished his pro career in 1975 with the Portland Thunder of the soon-defunct World Football League (WFL), where he completed 158 of 272 passes for 1742 yards and 11 TDs and 12 picks. Primarily a reserve as a professional, Horn passed for 3,369 yards and 20 touchdowns in eight NFL seasons.
After his playing career ended, Horn entered the real estate business in Colorado.
During a conversation in 2008, Horn talked about the excitement he felt when he was drafted by the Green Bay Packers as their first round draft choice with a contract for $15,000 in 1967. As he stated, "that was over $1,000 a month, something today's players wouldn't even cross the street for."
Packer head coach Vince Lombardi told his players that he was aware "three or four of you are here for the money and are sorry souls." Horn responded that the opposite is true today, that only "three or four are playing now for the love of the game."