Having come under fire following the 5-1 defeat at Burnley on Tuesday, the former England manager will have been feeling slightly happier when Ishmael Miller put them on course for a first home win of the season.
But Birmingham, back in domestic action after their Europa League win over Maribor, hit back in the second half through Chris Burke and Chris Wood, who scored twice to leave the Forest fans calling for the head of McClaren, as well as the club's board, come the final whistle.
The Burnley defeat was the latest in an already long line of setbacks for McClaren, who has also seen his side lose 4-1 at home to West Ham and 2-1 to Derby.
Those in attendance saw Forest enjoy the majority of the possession and the chances, with Miller, Matt Derbyshire and substitute Robbie Findlay all impressive.
But Birmingham, who won 2-1 in Slovenia on Thursday, were a danger on the break - something they demonstrated early on when Burke volleyed wide when well set.
Keith Fahey should have done better than nod Liam Ridgewell's headed centre wide after 28 minutes, as Birmingham enjoyed the better of the opening stages but neither side could create an opportunity of any real significance.
But that all changed after 36 minutes, as the home side took the lead.
Lewis McGugan whipped in a cross with pace and venom to the near post, Derbyshire headed the ball on and found Miller, who nodded home.
The goal invigorated the hosts, and they camped in Birmingham territory, forcing a succession of corners until the break.
And Forest took that momentum into the second period, with Miller and Derbyshire terrorising both Birmingham full-backs, and McClaren's side seemed to be starting to find some form of self-belief.
The visitors looked dangerous only on the counter-attack, but the Blues were struggling to find a killer ball in the final third.
And once again it was the home side who had the next sight of goal.
Radoslaw Majewski picked up a terrific pass from McGugan, skipped past his marker and took on the shot from a narrow angle but fired wide when perhaps crossing was the better option.
The hosts continued to dominate, Marcus Tudgay heading a McGugan free-kick wide and Findlay, on for Miller, also probing for an opening.
Time seemed to be running out for Chris Hughton's Birmingham, who were finding it hard to put together a period of possession, but the Blues had scored in each of their seven previous Championship outings this season and they extended that run with 15 minutes remaining.
First Burke seized on a loose ball in midfield and smashed a 25-yard drive past Lee Camp in the Forest goal.
And it then went from bad to worse for Forest moments later as Wood, on as a substitute, received Burke's cut-back and pulled the trigger, with a deflection looping the ball over the stranded Camp.
Wood struck again with two minutes left, breaking through the hosts' backline to slide home with consummate ease and wrap the game up in the visitors' favour.