Goalkeeper Thompson and midfielder Walker, both 18, have signed one-year deals after progressing through the club's academy.
"This is a very early stage in their careers and it's now up to them to make further progress from being a development squad player to earn a place in the first-team squad proper," manager Steve Evans told the club's official website.
"I would expect both lads to have some involvement in our lesser profile pre-season matches."
Thompson has not yet appeared for the first team, but has been a regular in the matchday squad, while Walker made his Millers debut last season as a late substitute against Cheltenham and stepped off the bench in the FA Cup victory at Notts County and the home win over Accrington.
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