The Panthers acquired Darnold from the New York Jets after trading this year's sixth-round pick, along with their 2022 second- and fourth-round selections.
After landing Darnold, Carolina has traded former starter Teddy Bridgewater and passed on drafting Justin Fields and Mac Jones. The Panthers instead selected cornerback Jaycee Horn on Thursday.
"I'm not going to talk about other players; we brought Sam here for a reason," head coach Matt Rhule said. "We're excited to see what he can do. We're excited to give him an opportunity.
"We traded for him because we believe in his potential."
Darnold spent the first three seasons of his career with the Jets after being selected with the third overall pick in the 2018 draft.
Carolina also exercised wide receiver D.J. Moore's fifth-year option.
Moore recorded a career-best 1,193 receiving yards in 2020. He also posted 66 receptions for four touchdowns. The 24-year-old has recorded back-to-back 1,000-yard seasons.