Cotterill joined the club on Friday evening after Forest agreed a compensation package with Portsmouth.
"I'm really excited about the appointment," Clark told the club's official website.
"Steve was always at the top of the list and he fits the criteria that we set out when I first started looking at this with the board last week.
"He's enthusiastic, he's hungry, he's got a terrific CV and he's experienced at this level. He was everything we were looking for."
Cotterill will officially start his new job on Monday, with assistant manager Rob Kelly taking charge of Forest's match with Peterborough United this weekend.