Marginal mandibular branch of facial nerve lies in:
Branches and Distribution of facial nerve:
Within the facial canal

Greater petrosal nerve
The nerve to the stapedius
The chorda tympani

At its exit from the stylomastoid foramen

Posterior auricular
Digastric
Stylohyoid

Terminal branches within the parotid gland

Temporal
Zygomatic
Buccal
Marginal mandibular
Cervical

Key Concept: 
The marginal mandibular branch of the facial nerve is found superﬁcial to the facial artery and  (anterior) facial vein. Thus, the facial artery can be used as an important landmark in locating the  marginal mandibular nerve during surgical procedures.
Subdermal fat plane
Subplatysmal plane deep to facial vein
Platysma plane
Subplatysmal plane superﬁcial to facial vein