Sergeant Major Mlachak enlisted in the Marine Corps in July 2001 and reported to recruit training at Marine Corps Recruit Depot, Parris Island, SC.  After graduation he reported to Infantry Training Battalion, Camp Geiger, NC where he obtained the military occupational specialty (MOS) 0311, Rifleman.  

In December 2001, Private Mlachak reported to 3D Battalion, 8th Marines, Camp Lejeune, NC where he served as a Rifleman, Fire Team Leader and Squad Leader.  In January 2002, he was promoted to Private First Class.  He deployed to Afghanistan in support of Operation ENDURING FREEDOM from July to September 2002 and was promoted to Lance Corporal during that time.  From December 2002 to September 2003, LCpl Mlachak deployed to Okinawa, Japan.  From March to June 2004 he was deployed to Haiti in support of Operation Secure Tomorrow.  While deployed he was meritoriously promoted to Corporal.  

In November 2004, Corporal Mlachak reported to Weapons Training Battalion, Stone Bay, North Carolina where he served as a Block NCO, Pit and Tower NCO, Primary Marksmanship Instructor, and Line SNCOIC. He obtained the additional MOS's of  Range Coach (8530) and Primary Marksmanship Instructor (8531).  Corporal Mlachak was meritoriously promoted to Sergeant on 2 August 2005.    In August 2007, Sergeant Mlachak reported to 3D Battalion 2nd Marines, Camp Lejeune, NC where he served as a Squad Leader, Platoon Sergeant,Weapons Platoon Sergeant, and Platoon Commander.  From October 2007 to May 2008 he deployed to Iraq in support of Operation IRAQI FREEDOM.  He then deployed with the 22nd Marine Expeditionary Unit from May to December 2009.  While deployed he was promoted to Staff Sergeant.  From January to April 2010, SSgt Mlachak deployed to Haiti in support of Operation Unified Response.   In January 2011, Staff Sergeant Mlachak reported to Marine Corps Recruit Depot, Parris Island, SC for Drill Instructor Duty.  

Upon graduation he was assigned to the 3D Recruit Training Battalion where he served as a Drill Instructor and Senior Drill Instructor.  During his tenure, he was assigned to Instructor Training Company and served as a Marine Corps Instructor of Water Survival and Marine Corps Instructor Trainer of Water Survival.   In May 2014, Staff Sergeant Mlachak reported to 3D Battalion, 2nd Marines, Camp Lejeune, NC where he served as a Platoon Sergeant, Weapons Platoon Sergeant, and Company Gunnery Sergeant.  From February to August 2015 he was deployed to Okinawa Japan.  In September 2015 he was promoted to Gunnery Sergeant.  From August 2016 to January 2017 he was again deployed to Okinawa, Japan .  

In April 2017, Gunnery Sergeant Mlachak reported to NROTC, Miami University, Oxford, OH where he served as the Assistant Marine Officer Instructor.  While there, he augmented three summers at Officer Candidates School and served as a Candidate Platoon Sergeant and Academics Instructor.    In October 2019, First Sergeant Mlachak reported to Weapons and Field Training Battalion, Edson Range, Camp Pendleton, CA where he served as the Headquarters Company and Range Company First Sergeant.   In November 2021, First Sergeant Mlachak reported to I MEF Support Battalion, Camp Pendleton, California where he served as the Headquarters Company First Sergeant.  In April 2024 Sergeant Major Mlachak was promoted to his current rank and reported to Combat Logistics Battalion 26 where he currently serves as the Command Senior Enlisted Leader.    Sergeant Major Mlachak's personal awards include the Navy and Marine Corps Commendation Medal with three gold stars, Navy Marine Corps Achievement Medal with two gold stars, and the Combat Action Ribbon.