A late lean injection combustor assembly may include a first interior in which a first fuel supplied thereto is combustible, a flow sleeve annulus including a second interior in which a second fuel supplied thereto is combustible, at least one fuel injector disposed about the second interior, and at least one elongate premixing conduit disposed about the flow sleeve annulus and in fluid communication with the at least one fuel injector. The at least one elongate premixing conduit may be in fluid communication with a compressor discharge air and the second fuel such that the compressor discharge air and the second fuel are premixed within the elongate premixing conduit before entering the second interior by way of the at least one fuel injector.