tmux exits with [exited] on mac os x

I had this same problem. It was caused by having set-option -g default-command "reattach-to-user-namespace -l zsh" in my .tmux.conf without having reattach-to-user-namespace installed.

The fix was to install "reattach-to-user-namespace" via Homebrew (brew install reattach-to-user-namespace)

In my situation I had been fiddling with a number of dotfiles so expected things to be amiss. My fix happened to be shutting down tmux with killall tmux. After this I was able to spin up properly.

Do make sure that the default-shell option only contains the executable path and does not contain options.

In /etc/tmux.conf or ~/.tmux.conf

set-option -g default-shell "/bin/bash"
set-option -g default-command "bash -l"
set-option -g default-path $HOME
set-option -g default-terminal "screen-256color"