Military Justice Improvement Act of 2013 - Amends the Uniform Code of Military Justice (UCMJ) to direct the Secretaries of Defense (DOD) and Homeland Security (DHS) to require the Secretaries of the military departments to modify the process for determining whether to try by court-martial a member accused of: (1) certain UCMJ offenses for which the maximum punishment includes confinement for more than one year; or (2) a conspiracy, solicitation, or attempt to commit such offenses.

Requires courts-martial determinations for such offenses to be made by a commissioned officer available for detail as trial counsel who is outside the chain of command of the member subject to the charges. (Currently, courts-martial decisions are made by commanding officers of accused members.)

Excludes from the revised procedures that require counsel outside the chain of command to make such courts-martial determinations certain UCMJ offenses relating specifically to military service, including absence without leave, insubordination, and aiding the enemy.

Prohibits an outside counsel's determination not to proceed to trial by general or special court-martial from altering the authority of commanding officers to: (1) refer noncapital offenses to summary courts-martial with limits on the punishments that may be imposed; or (2) impose non-judicial admonitions, reprimands, or disciplinary punishment for minor offenses.

Requires military chiefs of staff to establish offices to convene general and special courts-martial. Prohibits officers in the chain of command of an accused or a victim from detailing members to serve on a resulting trial by court-martial.