Translation:My father worked in his office every Monday.
I hear you, and I take it that idiomatically in French one can only work "dans un bureau", never "à un bureau".
In (American) English there really isn't a difference between working "at an office" or "in an office". If anything, as a native speaker I'd say working "in an office" implies there's some alternative like being a traveling salesman or working outdoors.
tous les lundis = "every Monday".
"all mondays" is an incorrect translation as well as being grammatically poor English.