The former Scotland and Rangers manager replaced Sean O'Driscoll as coach on December 27 and presided over four defeats in seven games.
Trouble bubbled to the surface during last month's transfer window when Peterborough striker George Boyd was expected to join Forest, but the deal collapsed due to the player having an eye defect.
It is understood that McLeish still wanted to press ahead with a permanent transfer, but the Al-Hasawi family, Forest's Kuwaiti owners, refused and were willing only to sanction a loan deal.
Boyd was one of three players that McLeish had targeted on deadline day. He had also hoped to bring in Stephen Ward from Wolverhampton Wanderers and Chris Burke from Birmingham City, but all three deals failed to materialise.
There were rumours that the 54-year-old had resigned the following day, but he remained in charge and oversaw a 2-1 weekend defeat to his former club Birmingham in which Burke scored a double.
However, the tensions at the club finally gave way this morning as McLeish parted company with the club by mutual consent.
He said: "I am extremely saddened to be leaving after such a short period.
"Both parties entered into the relationship in good faith, however, there was a difference in the understanding of the development strategy of the club."