Siemian was born on December 26, 1991, in Danbury, Connecticut. He is a quarterback in the NFL.
He attended Olympia High School in Orlando, Florida. On the football team, he threw for an Orange County record of 6,144 career yards and 53 touchdowns as quarterback for three seasons with the Titans. He was also an outstanding baseball shortstop, third baseman, and outfielder, and was voted Mr. Olympia by his classmates during his senior year.
He played college football at Northwestern University; in 2010 he did not see action. In 2011 he was a Backup quarterback who played in eight games, completing 16-of-26 passes for 256 yards and three touchdowns on the year.
In 2012 he threw for 1,312 yards on 128-of-218 passing (58.7 percent) with six touchdown passes. In 2013 he Ranked sixth in the Big Ten in completion percentage (59.7), seventh in the conference with 12.07 yards per completion and ninth in the Big Ten in total offense at 181.8 yards per game. He Threw for 2,149 yards and 11 TDs and Appeared in all 12 games. In 2014 he was team co-captain and started first 11 games; he earned Academic All-Big Ten.
He was drafted with the 250th pick (overall) of the 2015 NFL Draft by the Denver Broncos. Siemian made his NFL debut against the Pittsburgh Steelers on December 20, 2015. On February 7, 2016, Siemian was part of the Broncos team that won Super Bowl 50.
On September 25, 2016, Siemian became the first quarterback in NFL history to throw 300 yards and completed four touchdowns with no interceptions in his first career road start. He compiled an 18-10 touchdown-to-interception ratio and an 84.6 passer rating.
On March 19, 2018, Siemian, along with a 2018 seventh-round draft pick, was traded to the Minnesota Vikings; he saw no action in the 2018 season. On March 20, 2019, Siemian signed a one year deal with the New York Jets.
In march of 2022, he signed a two-year deal with the Bears.