I would suggest this, if I may.
 
His first job was baking bread.
He first worked as a baker.
 
Why?
The job title is  �baker �.
The job / work is baking bread
 
Perhaps even this: His first job was as a baker. (He worked as a baker.)
 
The way you phrased it, it sounds as though the  �job � is a person.
The job name cannot be the person. 
 
Although we often say it in casual speech as you wrote it, neither is correct, strictly speaking.
Would another person correct me if I am wrong, please?
Hope that helps.