Demand for petrol has dropped since the US attacks
The news brings the firm's recent run of reporting ever-increasing profits to an abrupt end.
Last year, the company made $17.7bn, more money than any US corporation in history.
It is the first decline in quarterly profits since the giant was formed in November 1999 through Exxon's purchase of Mobil.
The profits of energy firms are extremely dependent on the prices of oil, natural gas and electricity.
Opec's broken promise
Profits at the Texas-based company fell to $3.32bn in the three months to the end of September, down from $4.29bn in the same period a year earlier.
The group's chairman, Lee Raymond, said results were hurt by an "adverse economic and commodity price environment".
The global economic slowdown has curbed demand for fuel, while the 11 September terrorist attacks have slashed demand for jet fuel.
The company also blames the oil cartel Opec for producing more oil than it promised and sending crude prices lower. However, speculation remains that Opec will cut production at its next meeting in November.
Safe haven?
Profits at the firm's exploration and production business, which depend most heavily on the wholesale crude oil price, fell over 30% to $2.13bn.
And with many market observers saying that Opec has now lost its grip on price controls, analysts are predicting crude oil prices to fall further.
But while the oil companies may see their profits fall, the energy sector is holding up far better than many other companies affected by the economic downturn.
And industry heavyweights such as ExxonMobil are often a safe haven for investors who fear that smaller companies are more vulnerable to bankruptcy.