Appointed defence minister on 24 June 2002, the military commander was the former leader of the Northern Alliance that battled with US forces to oust the Taliban government.
He is now one of four interim vice presidents and represents the most powerful military force after the US and NATO.
The marshal continues to maintain a private army and independence from the national government, benefiting from his vast experience in combat and military affairs.
Fahim was born in Ommerse, a village in the Panjshir Valley, in 1957.
Travelling to Kabul, he went to high school there and remained until the communist coup in 1978.
During the Soviet invasion of Afghanistan, he was a member of KHAD, the secret police of the communist government.
In time, he switched sides and decided to join the Islamic uprising against the communists in the late 1980s.
Returning to Panjshir, he began to work under mujahidin leader General Ahmad Shah Masud – a position he maintained for nearly 20 years.
The Masud years
Under Masud, Fahim played an important role in the resistance to Najib Allah's Soviet-sponsored government in the north, a revolt which culminated in the downfall of the communist regime in 1992.
Appointed Minister of National Security to the mujahidin government in the same year, he also served as the commander of the state's forces in Kabul for four years before the Taliban began to enjoy their military successes.
After the fall of Kabul to the Taliban in 1996, Fahim was appointed commander of anti-Taliban forces north of Kabul before being transferred to the north.
Swapping the uniform for a suit,
Fahim (R) adopted a political role
During these years of struggle, he acted as the commander and administrator of Jamiat Islami forces in Parwan, Baghlan, and other areas and served as the deputy director of the political committee of Shora-i-Nizar led by General Masud.
He was constantly moved around different fields of operation. Rumour has it that the reason for this was because of his reputation for being easily corrupted, especially by his brothers.
By keeping him constantly on the move, however, General Masud may have reckoned that it was less likely that Commander Fahim could build any permanent alliances.
When Masud was assassinated by supporters of the Taliban on 9 September 2001, Fahim was the natural choice to take up the leadership.
He was the only commander with experience on all the different fronts against the Taliban.
Until August 2004, Fahim represented the power behind the throne.
Military strength remains
Not much can happen in the former Northern Alliance without his tacit approval. Even the so-called national army comes under his effective control, in addition to his own private forces.
Several assassination attempts have been made in recent years.
The last car bombing, in April 2002, missed its man by a matter of seconds.
Many political analysts have argued throughout 2003 and 2004 that unless the US or the wider international community are prepared to face Fahim down, the chances of a national Afghan government forming any time soon are very limited.