Automapper - Mapping a nested collection to a list
When dealing with collections, the trick is always to set up a mapping of the items in the collections and never (well, I've never seen a case that needed) a mapping from one collection to another. That's the point of the generic.

The trick I always use is to work backwards, starting with a mapping between the types inside the collections and then going up a level. If one object needs to go up 2 levels, it's time to use ForMember to get to a nested property.

What you want to do in this case is to set up the following mappings:

Mapper.CreateMap&lt;SourceObject, TargetObject&gt;();
Mapper.CreateMap&lt;SourceOuterObject, TargetOuterObject&gt;()
    .ForMember(dest =&gt; dest.TargetList, opt =&gt; opt.MapFrom(src =&gt; src.SourceSet.SourceList);