MOHS surgery: incision reopened, will it ever close?

I'm not sure where your skin cancer was removed- but it will heal from the inside out. Keeping the area clean and covered with vaseline will help things fill in. If it's in a hair bearing area like the scalp then it won't fill in with hair bearing skin but there will be a bald spot in it's place. 
Keep cleaning and keep vaseline covered in and let your dermatologist or plastic surgeon know that it came apart.

Hope this helps and God Bless, 
Dr. Robb

Without photographs it's impossible to comment on your particular case. Healing by secondary intention is a legitimate way of handling the wound. Scar revision however is often necessary.

Thank you for your question.  Having a wound open after sutures have been
removed, known as dehiscence, can be a complication after Mohs surgery or any
surgery.  However, from your question it is not clear if perhaps the wound
created by the existing skin cancer was too large to close completely.  Regardless, it has been my experience that
when a wound heals on its own after dehiscence, it heals quite well as long as
it is properly cared for.  In similar
situations I’ve handled, patients who have kept the area clean and who have
applied a thin layer of Aquaphor or Vaseline to the area have experience
exceptional results.  I usually have my
patients follow-up every 1-2 weeks to evaluate healing.  Sometimes extra steps need to be taken to
speed up proper healing.  If you still
have concerns, please follow-up with your Mohs surgeon.  Good luck to you.

Thank you for your question.  I completely understand your concern but it is still early.  It can take weeks to a couple months to close on its own.  Healthy granulation tissue needs to form from the bottom up and unfortunately, this takes time.  Please continue to clean the wound and apply the ointment as prescribed.Best.Ariel Ostad MD PC 

When a wound which has been sewn up opens, it is called dehiscense, and can be a complication of any type of surgery. When it occurs, the surgeon usually has two options. One of these is to allow the wound to heal on its own. This is called second intention healing. This is usually accomplished by cleaning the wound at least daily, not letting a scab form, and keeping the wound lubricated with an ointment or Vaseline and keeping the wound covered between cleanings. This allows the wound to fill with granulation tissue from the bottom up. When it reaches the top of the wound, new skin will cover the top of the wound from the outside in. The result is a flat and smaller wound then the original one. Letting it open to air will cause a scab to form, preventing the wound from filling with granulation tissue and forcing new skin to form under the scab. This can result in a depressed wound close to the same size as the original.The surgeons second option is to re-close the wound. This is dependent on a number of factors including blood supply, laxity of surrounding tissue, infection, new skin forming over the wound edges, etc. The minimum required prior to re-closing a dehisced wound is to "freshen-up" the wound edges. This is removing any epidermis which has formed over the wound edges preventing them from attaching to one another.