Manager Mark McGhee announced he was "absolutely delighted" that 22-year-old Clarkson and 20-year-old Reynolds had made new commitments to the club.
Clarkson has signed a new two-and-a-half-year deal, keeping him at Fir Park until 2010.
Scotland Under-21 international Reynolds has agreed a one-year extension with the club, and his contract now expires in 2011.
McGhee told motherwellfc.co.uk: "I am absolutely delighted that we've got both lads to commit on a longer-term basis.
"We're trying to build a good, young dynamic squad at Motherwell and Reynolds and Clarkson have been integral to what we've achieved so far."