Fewell replaces Raheem Morris, who is expected to join the staff of the Atlanta Falcons.
Fewell was fired by the Giants earlier this month. He spent five seasons in New York and was the interim head coach of the Buffalo Bills for seven games in 2009.
Clark spent the last two seasons as the strength and conditioning coordinator for the Chicago Bears. He replaces Ray Wright, who was let go after five seasons with the Redskins.
The Redskins announced the moves Tuesday.
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