The move extends Mayfield's contract in Cleveland through 2022, when he'll make $18.86 million.
The team also exercised the fifth-year option on cornerback Denzel Ward, the No. 4 overall pick three years ago.
"Both of those players have really done a nice job for us over the first three years," general manager Andrew Berry said Friday. "We view both of those guys as young players who continue to ascend."
Last season, Mayfield -- the No. 1 pick of the 2018 NFL draft -- quarterbacked the Browns to their first playoff victory in 26 years. In that 48-37 win over the Steelers, Mayfield passed for 263 yards and three touchdowns as the Browns snapped a 17-game losing streak in Pittsburgh during their first playoff appearance since 2002.
Mayfield finished last season ranked 10th in the league in QBR, with 26 touchdown passes and only eight interceptions. From Week 7 to Week 15, he was actually third in QBR, trailing only last season's MVP (Aaron Rodgers) and the 2019 MVP (Patrick Mahomes).
Coming off a season in which he broke an NFL rookie record with 27 touchdown passes, Mayfield struggled at times in 2019, as the Browns finished a disappointing 6-10. But playing for a fourth head coach in his third season, Mayfield thrived under first-time head coach Kevin Stefanski last year.
"I hope it is significantly better," Stefanski said this week, when asked about Mayfield improving with another season in his scheme. "He has heard these plays over and over again. We have streamlined concepts, and we have tried to adjust what we do based on his strengths and our players' strengths. I would hope he is much more comfortable from that perspective. That is what time allows you to do -- time together."
When he took over, Mayfield became Cleveland's 30th starting quarterback since the franchise returned to the league in 1999.
Mayfield is also eligible for an extension, which the team could engage him on later this offseason.