The benching comes on the heels of Mills' worst performance of the season Sunday in a 23-10 loss to the Washington Commanders. Against the Commanders, Mills completed 57.6% of his passes for 169 yards and two interceptions, including a pick-six on the opening drive.
Texans coach Lovie Smith had declined to name a starting quarterback during his Wednesday news conference, saying he didn't want to "give the opponent an advantage."
NFL Network first reported that Allen would start against the Dolphins.
The Texans had hoped Mills, a 2021 third-round pick out of Stanford, could become the quarterback of the future after showing promise in his rookie year. But he has struggled through his sophomore year, with his completion percentage falling to 61.9% and his passer rating dipping to 78.1, ranking in the bottom 10 in the NFL in both categories.
Mills has also had an NFL-high 11 passes intercepted, surpassing his interception total (10) from last season.
The Texans hope the switch to Allen will provide a spark to an offense that ranks 32nd in total yards per game (285.9) and 26th in passing yards per game (192.2).
This is Allen's fifth season in the NFL after spending his first four years with the Carolina Panthers and Commanders. He is 7-10 as a starter with 24 touchdowns and 17 interceptions in his career.