The Forbes list, based on money made by Idol alums between 2011 and 2012, puts the 'Stronger' singer at the top of the tree with an estimated income of $8 million.
Clarkson, who recently sang at President Obama's inauguration, was helped to the figure with her platinum-certified albums, television appearances, tour and commercial for Toyota.
Carrie Underwood and Chris Daughtry share the second spot with estimated earnings of $5m each, while Scotty McCreery came in next with an estimated $4m.
[Left: Carrie Underwood / Right: Chris Daughtry]
Rounding out the top five is Jennifer Hudson, who helped boost her earnings with a Weight Watchers endorsement and her memoir I Got This.
Last year's American Idol winner Phillip Phillips does not make the top ten as his success came too late for the list, though Forbes suggests that he could break through next year.
The list is based on estimated income between May 2011 and May 2012, and does not take into account taxes, the cut given to American Idol, and fees paid to agents and managers.
The top 10 American Idol earners last year are:
1. Kelly Clarkson ($8m)
2. Carrie Underwood & Chris Daughtry ($5m)
4. Scotty McCreery ($4m)
5. Jennifer Hudson ($3m)
6. Jordin Sparks ($2m)
7. Adam Lambert ($1.5m)
8. Katharine McPhee & Kellie Pickler ($1.1m)
10. David Archuleta & David Cook ($1m)