Function to open Marked.app from Emacs showing the contents of the current Markdown file.

Open Marked.app from Emacs.el

(defun markdown-preview-file ()
  "run Marked on the current file and revert the buffer"
  (interactive)
  (shell-command 
   (format "open -a /Applications/Marked.app %s" 
       (shell-quote-argument (buffer-file-name))))
)
(global-set-key (kbd "C-c m") 'markdown-preview-file)

Thanks for getting me started down the path of what I wanted to accomplish here. I added some behavior to only load Marked.app if it is installed; otherwise, fallback to regular markdown-preview

(defun marked-markdown-preview ()
  "run Marked on the current file if Marked is installed;
otherwise fallback to markdown-preview"
  (interactive)
  (let ((marked-app "/Applications/Marked.app"))
    (if (file-exists-p marked-app)
        (shell-command
         (format (concat "open -a " marked-app " %s")
                 (shell-quote-argument (buffer-file-name))))
      (markdown-preview))
    ))

(eval-after-load 'markdown-mode
  '(progn
     (define-key markdown-mode-map (kbd "C-c C-v") 'marked-markdown-preview)
     ))