How can I get a more defined Jaw line? Can TMJ effect the appearance of my jaw line? (photos)

You have two issues with your profile: One is the double chin you mentioned. The other is that your chin is a little weak. Generally, chins look most balanced when they are in line with your lips; yours is recessive.
To give you a better profile, I'd recommend facial liposuction or Kybella to remove your double chin, plus a chin implant. Chin implants are easy to insert under local anesthesia in the office. They can last for years or a lifetime.
As for the TMJ, Botox injections could relax your masseter muscles and give you some relief. The TMJ shouldn't affect the look of your profile. However, once you get Botox in the masseters, it will slim down the look of your face a bit. I hope this helps.

Without an exam your question cannot be answered.  You will need to see an Oral/Maxillofacial surgeon to see if othognathic surgery, genioplasty or using an implant is best for your specific case. It should be noted that patients with severely weak jaws or abnormally functioning jaws may be better treated with other facial surgery (orthognathic surgery), although a chin augmentation may be an option at a later time. These patients should seek the advice of a plastic surgeon to determine which course is right for them. Here is some information to help you make this decision: Candidates for Chin Augmentation - Patients whose chin profiles are weak but have normally functioning jaws are candidates for Chin Augmentation. Many of these candidates have used injectable fillers in the past to strengthen the protrusion of their chins, but are looking for a permanent solution. If you are one of these patients, then you are a likely candidate for chin augmentation
#weakchin #chinimplant

There is no correlation between your TMJ issues and the soft tissue fullness along your jawline. Liposuction contouring of your jawline would be the best approach to try and get some better definition.